Enemy Territory: Singleplayer by William Faure is a custom campaign still in active development that aims to form a cohesive SP experience based on the MP game of the same name by Splash Damage (2003). As of v2.00 (June 30th, 2024), it features a total of seven missions spanning 18 levels, as well as 30 achievements to unlock.
This guide covers all of the said achievements (alongside additional secrets) and details them on a mission-by-mission basis. Here are a few things to note before starting out:

The achievements that are not directly tied to specific missions are detailed in this section.
Even though you start every mission with your binoculars, an extra pair can still be found laying around in each of them. Picking up one of those grants you this achievement.
As of RealRtCW v5.0, the following four achievements can also be unlocked in an OG session with expanded arsenal enabled, as the weapons in question are added to the base game as well:
Choose Engineer as your class, attach a grenade (alt. fire) to your M1 Garand and fire away.
Start as a Field Op and throw a smoke signal on any group of enemies while you are outdoors.
Playing as a Medic, make good use of your gas grenade and wait for your enemies to fall.
Soldier class has the privilege of taking a Browning M2 to the missions, yet Venom guards are hard to come by. Be on the lookout for them in Würzburg Radar (Part 3) or Fuel Dump (Part 3) while playing as a Soldier and make them say hello to your not-so-little friend.
Replay and complete a mission of your choice on any difficulty level with one of the modifiers enabled. Of the four ones on offer (No HUD, No Pickup, No Manual Save and 999 Mode), 999 Mode gives you the easiest time (i.e. that number of HP and ammo - without any penalties).

August 21, 1942 / Recommended Class: Soldier or Engineer. Two secrets in three levels.
Siwa Oasis, Part 2 - Three pieces of treasure are hidden in a locked room on the second floor, next to the wall you need to destroy. To access the room from the other way around, go just a little back and climb the ladder near one of the two inner wells. From there, jump across to the topside of the damaged wall and follow it to the other side. Utilize the wooden supports that jut out from the side of the next dwelling to make your way to the top and to the door ahead.
Tip - If you do not want an all-out war in the garrison (at least until the cavalry shows up), try to clear out the alcoves and the side rooms with quick Sten kills. With fast enough reflexes, you can get rid of most of the enemies in the first half without alerting the others.
Siwa Oasis, Part 3 - A treasure chest is hidden in an inaccessible room next to the MG on the second floor of the garrison. The room has three windows on each side. Once you reach the MG, shoot the plank that holds the door on the other side or throw a grenade through the closest window to destroy it so that you can get into the room through the door as usual.
Unlocks upon completing the first mission for the first time, regardless of the difficulty level.

October 10, 1942 / Recommended Class: Engineer or Sniper. Two secrets in three levels.
Seawall Battery, Part 2 - The titular battery is to be found in the latter half of this map. You have to be pretty quick on your toes to blow it up in the shortest amount of time possible.
Get some dynamite from the cabinet in the MG post overlooking the courtyard and destroy the generator as ordered to open up the way to the battery. Make a manual save right before you descend to the lower floor in case your upcoming rush fails. Equip a weapon that makes quick work of enemies, preferably an assault rifle or a heavy one.
The countdown starts as soon as you step into the lower floor. Take the stairs to the left and dispatch the black guard that stands in your way. On the second floor, ignore the flametrooper that rushes at you and run to the stairs to the left. Either run past or deal with the trio on the stairs and jump over the railing on the third floor to find yourself next to the battery controls. Drop down a bundle of dynamite and get to a safe distance.
Seawall Battery, Part 2 - There is a locked room housing a bunch of goodies on the third floor of the underground bunker. On your way out, open up the middle locker from the trio of lockers to the right of the door and pull the lever found inside to gain access to the room in question.
Seawall Battery, Part 3 - There is a locked floor safe in the barracks room of the next bunker.
Return to the initial area of this map and go through the trapdoor found on the floor. Pull the lever located on the wall at the other end of the room beneath. Back in the barracks room, get the four pieces of treasure residing within the now unlocked safe.
Unlocks upon completing the second mission for the first time, regardless of the difficulty level.

December 22, 1942 / Recommended Class: Sniper. Two secrets in a single level.
This mission can be dealt with either a stealthy approach or Blazko style. The path outlined below shows the quickest way of doing a stealth run. Mind that the Nazi officer who is crucial for the stealthy approach tends to get stuck on the path he follows. Once that happens on a particular run, even restarting from the autosave or a quicksave results in the same outcome. If returning for the achievement, be sure to get a fresh start by directly selecting the level from the main menu. Also, do not save or load your game in any way until the officer in question reaches his destination as doing so increases the chances of him getting stuck.
Upon start, take the path on the right. Drop down the solitary guard on your path with either the knife or Sten and continue on through the opening between the buildings directly ahead.
Make a slight left on the next street, take down the guard and reach the stairs to your left a little ahead. Note that the guard in question will be to your right patrolling down the street if you move to this area quickly enough. Wait beneath the stairs until the guard patrolling the tunnels shows up, take him down with Sten and check the next corner.
Kill the guard patrolling in front of the command post without getting noticed by the other patrol across the courtyard. Wait for this second guard to turn his back to you and make haste to the door at the corner on the other side.
Once you are in his quarters, you get a radio message and the officer starts moving. Take care not to be seen by him and make sure he turns the corner and goes out through the front door. There is a chance that he gets stuck even without leaving the quarters - if that happens, reload the mission from the main menu. You are expected to follow him but there are guards on his path and saving / loading as you are doing so may get him stuck at the front door of the bank.
The safer option is to simply wait it out in the quarters. He continues along his set path even without you going after him. Stand at the spot without saving or loading and exactly five minutes after he gets out, you will receive a radio message stating that he is inside the bank.
After the radio message, get the secret documents from the table, hit the checkpoint or save your game and step out through the door you got in. The destruction of the radio equipment and the command posts need to wait until you get inside the bank. Either down the patrol or sneak past him and return to the spot where you took out the second guard. From there, follow the lower pathway next to the central tower.
The courtyard in front of the bank is full of guards out in the open so it is best to avoid them altogether. Lean around the corner of the stairs and wait for the closest guard to get out of sight to the right. Before making your move, have a look to your left and kill the guard patrolling on the other side as he comes close to the tank barrier. Wait for your next chance, then jump down and rush towards the opening to the left. Take out the drunk guard and follow this back alley all the way around, taking another easy one out with Sten in the process.
As long as you are not spotted along the way, there should be three explosive barrels behind the bank. Blow them up and get in. Alarms sound at this point but you still get the achievement once you kill the officer and get the cache full of gold residing on the table.
Now that the heist is out of the way, you are free to go for the other objectives and the secrets. When you are done, return to the truck parked in front of the bank for the ride out of the mission.
There are eight pieces of treasure hidden behind the bookshelves in the officer's quarters. Destroy the painting on the wall and pull the lever to reveal them.
Another piece of treasure is hidden in a compartment in the alcove just below the courtyard where the tank and the officer are found. Push the brick that slightly juts out from the others in the alcove to reveal it.
ET v2.00: The alcove in question now also houses health and ammo cabinets.
Unlocks upon completing the third mission for the first time, regardless of the difficulty level.

March 31, 1943 / Recommended Class: Medic, Field Ops or Sniper. Three secrets in three levels.
Würzburg Radar, Part 1 - There is a locked room on the second floor of the villa that houses a bunch of goodies. Utilize the destroyed wall next to the villa gate to get on top of the left pillar. From there, jump across to the other side of the gate and move along the perimeter wall. Shoot the window on the right side of the villa and sprint jump through to access the room in question.
Note that it is better to access the room right away, as you first approach the villa rather than after the ensuing firefight. Considering Sten (found within) is your weapon of choice, you will have an easier time clearing the place without reinforcements arriving to assist the officers.
Three pieces of treasure are also hidden on the same floor. Following on from the previous secret, once you grab the goodies from the locked room, step into the adjacent one and break open the chest at the foot of the bed to reveal them.
ET v2.00: The primary objective is not to destroy the radar dishes (as before) but rather to steal their parts by way of interacting with the boxes found on their sides.
Würzburg Radar, Part 2 - There are three pieces of treasure to be collected in the radar grounds. Get into the workshop near the east radar and crawl underneath the tank to grab the bars of gold laying out in the open.
Unlocks upon completing the fourth mission for the first time, regardless of the difficulty level.

April 6, 1943 / Recommended Class: Soldier or Medic. Two secrets in two levels.
Rail Gun, Part 1 - Three pieces of treasure are hidden near the water tank found on the overpass between the depot yard and the area leading to it. Get on top of the overpass by climbing the ladder on the wall and turn the valve on the water tank to reveal them.
Another piece of treasure is hidden in the alcove next to the rail gun ammo cache, beneath the platform that has an MG placement on it. Crush the slab of rock and crawl in to get it.
Rail Gun, Part 1 - If you somehow do not get this achievement even with all the objectives ticked, be sure to get on the MG found on the platform next to the rail gun ammo cache to make two proto soldiers spawn. There should also be reinforcements arriving once you get the dynamites from the cabinet found to the back of the said MG.
The MG found within the central bunker also spawns two more proto soldiers once you use it, so better check that one out if you still lack the achievement even after clearing out the depot yard.
Unlocks upon completing the fifth mission for the first time, regardless of the difficulty level.

April 14, 1943 / Recommended Class: Sniper. One secret in four levels.
The first and the second parts of this mission require you to keep it stealthy. Take note of the achievement description: it is also possible to get it in the second part, but it is harder to do so since that map is full of guards in close proximity and you need to take them all down without alerting the others to successfully complete the mission. Therefore it is advised to go for the achievement in the first map: it is quicker, safer and most of the enemies are avoidable, not counting the fact that it is also easier to restart from this map in case of failure.
Speaking of which, the achievement requires you to do it in one go, i.e., with no loads along the way. Even restarting from the autosave counts as a second attempt, so be sure to get a fresh start when retrying, by directly selecting the level from the main menu.
Fuel Dump, Part 1 - Two paths are available: either take the path before you and reach the tunnels via the entrance at the end of the frozen riverbed or turn around, cross the lake via the bridge and enter the complex that leads to the tunnels via its side entrance. The path outlined below covers the latter, as it is quicker and more straight-forward than the former.
Upon start, turn around and move towards the bridge. Take down the guard patrolling on it with the knife and cross to the other side. Check on the next guard patrolling ahead and make your move to the far side of the nearby building once he turns his back to you.
Wait for the guard patrolling in front of the building to turn the other way and move towards the watchtower. Climb the ladder and kill the sniper with the knife. Optionally, if you are good with Sten in medium range, you can try to kill the guard patrolling the side entrance while you are on the tower. Either way, return to the building while keeping an eye on the guard from before and Sten him when you have the chance.
ET v2.00: So long as you play the Sniper class, you can make quick work of both of the patrols mentioned above by Snoop'ing them from the watchtower. The five guards by the gate can also be sniped as a precaution, considering you are good enough with the said rifle: doing so will make your trek through the following tunnels more comfortable.
Climb the antenna next to the building and get in through the chimney. Take down the officer inside and get the secret documents from the table.
Outside once more, check on the guard patrolling the side entrance and make your move towards the stairs to the right once he starts going the other way. Either kill or sneak past him and get into the complex.
Once inside, get on top of the railing found in the next room and wait for the guard to appear in the doorway on the lower floor. He spots you right away so make quick work of him with Sten. Descend to the said floor with Sten at hand and quickly get rid of the operator inside as you step into the adjacent room. Be sure to destroy the comms system on the table.
Here comes the tricky part. Continue down the stairs but take it slowly to not get spotted by the proto soldier patrolling the following tunnels. Lean around the corner and wait for him to get out of sight at the far end. As soon as he is out of sight, take the path to your left. Be sure to crouch as you move forward as not doing so alerts the guards on the other side of the tunnel gate. Make quick work of the guard waiting in the opening that connects the tunnels to the riverbed with Sten and hide in there.
Check on the proto soldier as he comes back to the side you are on. Once more wait for him to get out of sight. Move to the narrow pathway to your left when you are in the clear. Do not forget to lean around the corner before you jump into the back of the truck to not get spotted by the proto soldier in the final stretch.
Tip - Despite what is told on the objectives screen, it is not possible to complete the mission by directly going to the fuel dump and blowing it up - doing so triggers the alarm and results in a failure. You need to clear out the whole level using stealth-friendly equipment (knife, Sten and Snooper rifle) without getting spotted, before destroying the titular dump.
Fuel Dump, Part 2 - There is a locked compartment within the outer perimeter room overlooking the way to the complex. Destroy the noticeboard next to door that leads to the ramparts and pull the lever to gain access to the compartment in question.
Unlocks upon completing the sixth mission for the first time, regardless of the difficulty level.

June 30, 1943 / Recommended Class: Soldier or Fields Ops. Two secrets in two levels.
Warbell, Part 1 - More or less a progression achievement. Once the lengthy introductory chat of the team concludes (*), follow Agent Two and her team of three into the nearby caves. Shortly after Waters starts working on the damaged wall, enemies show up on the opposite side to keep you from accomplishing your goal. As soon as you kill the last of them, Waters blows up the wall and the achievement gets unlocked.
Tip - As your team plans the first step, you may want to get the Snooper rifle by the waterfall and take down the enemies in front of the guardhouse. Otherwise, it gets pretty hectic in there once you breach your way in.
Once you destroy the generator located on the third floor of the guardhouse, be sure to check the newly-revealed room on the other side for the sole secret of this level.
Tip - Best exit the level while holding a scoped rifle, in preparation for the next achievement.
Warbell, Part 2 - At the start of the level, Blavatsky can be seen standing on the ledge to the left. Zoom in on her and take the shot before she leaves the scene (you have around seven seconds to do so). Doing so results in an early success, granting you this achievement as well as 'Warbell' for completing the mission.
After reloading the autosave, it is best to get the sole secret of this level before moving on. First make your way to the very end of the ledge where Blavatsky stood before. Once up there, keep hugging the rocks while moving towards the ladder in sight and jump onto it once you feel near enough. After that, it is just a matter of hopping onto the adjacent balcony through the hole in the wall, where a cache of goodies as well as a third team member await.
Warbell, Part 2 - Once you step into the abbey grounds via any of the three paths available, a cutscene kicks in where Blavatsky resurrects Heinrich once more. Since one of your objectives is to 'stop the awakening ritual', you now need to confront him in the ritual chamber.
It is better to get the experimental weapons Venom and Tesla from the ground floor and cleanse the whole abbey first, destroying the the command post and the radio equipment in the process.
You may also opt to destroy the warbell mechanism at this point (which prevents Heinrich from calling the undead into the arena during the upcoming fight), yet the achievement requires you to fight him before touching the warbell, so it is best not to do so.
Warbell, Part 2 - Olaric's chamber is found directly across the ritual chamber. Confronting him is not required for the completion of the mission, but necessary for unlocking the achievement and getting ahold of the treasure item he drools over.
It is best to go for him after Heinrich but before completing all of your objectives (cause doing so instantly ends the mission). Replenish your health and stock up on ammo via the cabinets found at the entrance of the ritual chamber, then step into Olaric's to tackle him.
Unlocks upon completing the seventh mission for the first time, regardless of the difficulty level.
And that is all until M8: Ice gets released. Feel free to comment if you require further assistance.
Keep up to date with Faure's progress on the upcoming missions through ET's ModDB page.