(The points mentioned below are displayed on the Speedrun map in the Spoilers Tab)
A - You start here. This is the beginning of the game's second-to-last level (third-to-last if you're playing on Hard, and you're a vigorous explorer). This level is somewhat shorter than the last few levels, but it is also very challenging. Directly outside of the elevator you're in, you'll find a small carpeted hallway, and to your left are two doorways, head through the furthest one to the left; you'll need some supplies from here first.
B - This room contains two Backpacks and one of each weapon. You'll need all of these goodies to be able to get through this level in one piece. Area-of effect and rapid fire weapons will especially be useful here, as you'll be engaged in close-quarters combat for the duration. Grab everything you can and then head through the door in the center of the starting room. In the next room, you'll find five doorways surrounding the room. The first one, straight ahead leads to the level proper; the others lead to stock rooms with supplies. Be sure to visit them first, as you won't last long with meager supplies. You can visit the store rooms in any order you choose, so the order of the listings below is arbitrary. Just be sure to visit them all.
C - This room contains a single Stielhandt and Machine Gun ammunition. Grab all you need and head for the other storage rooms.
D - This room contains a single Stielhandt and Flame Thrower ammunition. Grab all you need and head for the other storage rooms.
E - This room contains a single Stielhandt and Pistol ammunition. Grab all you need and head for the other storage rooms.
F - This room contains a single Stielhandt (the fifth one if you've been following this list in order) and Sniper Rifle ammunition. Grab all you need and head for the other storage rooms (or if you're reading this in order, to the actual level itself).
G - When you first open the door, you'll notice a few things: First, there's a POW out in the open, ready to be saved, and second, that he's surrounded by an SS, an Officer, a Rottweiler, and a Guard. Engaging the enemy is simple, what isn't simple is engaging them and trying not to hit the POW while doing so. Since it's the second-to-last (or third-to-last) level, the enemy won't be holding back, and they've taken to dirty tricks. The POWs on this level are still worth a whopping 10,000 points, but it'll be much more difficult here to actually save them. There are exactly twenty POWs on this level, adding up to a grand total of 200,000 points! But you'll have to be careful, the POWs are your only source of points on this level, and when they're dead, the points they would have awarded you are gone for good (plus, there's a 10,000 points penalty for each one you kill). The enemies in this room should be simple enough to handle without killing the POW, but this room is a pale comparison of things to come.
H - This room spikes up in difficulty compared to the last one; although there are three POWs in this room, they're protected by an Officer, an SS, a Rottweiler, and an Ironclad Panzer. Although the first three enemies are nothing new, generally you're not forced to fight the Panzers in such tight areas, so dealing with the one in here and still managing to rescue the POWs can be troublesome. The best strategy is to wait for the enemies to come to you (as will the POWs). Typically, the Panzer and other enemies will collect near the entrance to the room (so will the POWs, but they disappear once they get close enough to be rescued). This gives you an excellent opportunity to use the Flame Thrower, Machine Gun, or even a Stielhandt or two (though, try to save the Stielhandts for later areas if possible). You should make sure to eliminate each enemy quickly, particularly the SS and the Officer. There's health laying around nearby the entrance, and you don't want them to steal the health that rightfully belongs to you.
I - This room has two POWs inside, protected by a Guard, a Rottweiler, an Officer, and an SS. Not much different from the first room really, save that you'll have to save two POWs, and you're coming in from an angle. Because of the nature of this room and the direction you came in, you'll likely be able to catch one or two of the enemies right away, but the others generally hide around the corner to the right. You'll have to engage them carefully if you want to save the POWs. Once that's dealt with, you'll notice a Red Key Door at the left end of the room. You can't enter inside it right now, but just know that there's a small supply stash inside, just for if you need it.
J - This room can be troubling. You'll immediately catch the attention of a Sniper when you open the door, easy enough to Knife, but to your right will be four POWs, a second Sniper, two Guards, a Rottweiler, an Officer, and an SS. All of the enemies will have to travel around a table in order to get you, so it's an easy spot to Stielhandt, but it's also a dangerous bottleneck for the POWs. You'll have be careful not to hit them as the various enemies make their way towards you. You'll also want to Snipe the second Sniper before he can get in a few good shots, putting you out of commission before the POWs can even reach you. Once all of the enemies in here are dead, and all the POWs are saved, check the back wall to the far left (after the first turn to the right), there's a supply cache hidden inside a secret there. Along the wall to the left of said ammunitions cache is another secret containing a Stielhandt. Be sure to grab all of these supplies if you're running low.
K - This room can be especially challenging; at the far end of the room is a Sniper that will most likely pepper you with shots while you deal with the enemies in the foreground. Said enemies include a Guard, a Rottweiler, an Officer, an SS, and an Ironclad Panzer. There are also two POWs in this room, so be careful when trying to go on the offensive. Trying to rush in to reach the Sniper is unwise, as a second one is standing to his right and will likely pick you off before you can reach the first one. Your only choice of action right now is to deal with the enemies in the foreground near the entrance to the room. If you can manage a shot from the Rifle at the first Sniper, then go for it. The sooner he's dead, the better. Once all of the enemies are dead, there are four health items in the center of the room to restore your wounds; since the last two rooms didn't have any health inside them, this should help ease the tension somewhat. There's also a secret to the left of where you came in.
L - Hidden away in an alcove at the far corner of the room is the Red Key, which you can use to open the door you found two rooms back. There are also stores of several types of ammunition sitting around in the small alcove, just in case you need any more ammunition.
M - This hallway is stocked with numerous foes. This is confounded by the fact that five POWs are running around amongst them. The hallway is protected by two Guards, two Rottweilers, two Officers, two SS, and an Ironclad Panzer. The best choice of action would be to lure all of the enemies inside this hallway into the previous room and utilize the cover present there. The rooms adjacent to the hallway in question each contain two fireplaces which themselves contain either a Medkit or a suit of Body Armor. The corridor to the right of the hallway also has a secret containing various types of ammunition inside.
N - This is the last room containing enemies or POWs inside it (aside from a secret area you've yet to visit). There are three POWs standing amongst a group of enemies, which include a Rottweiler, a Guard, an Officer, an SS, and an Ironclad Panzer. It shouldn't be too difficult to take care of all of the enemies in this room; simply luring them out into the previous hallway or even the room before that should aid you in dealing with them.
O - At the end of the room is an alcove, lined with various types of ammunition. Inside the alcove is also a Fireplace with the Yellow Key inside it. You don't need the ley to exit the level proper, but you do need it to access some of the secret areas, still lying in wait inside the level.
P - Back in the large dining room (Two rooms back from the room with the Yellow Key inside), is a secret area containing a Stielhandt inside. You may have already found and investigated this area, but there's also a Yellow Key Door inside that you wouldn't have been able to enter had you found this area earlier.
Q - Inside the Yellow Key Door is a small room with a Switch inside. Flip the switch to open a new passage in an area visited earlier in the level.
R - Behind this Red Key Door is a small storage room containing a Backpack inside. Since only a backpack is in here, you don't need to spend much time here for anything else.
S - Beyond the last enemy-infested room in the level is a room with some health items at either end and a pair of elevators. Inside the elevator to the right is a Backpack for your efforts.
T - This is the exit elevator. It will take you to the game's final level (or second-to-last level). Unless you want to find the secret elevator, this is your ultimate destination on this level. If you're ready, head on to Level 24: Adolf Hitler.
U - Back in the starting room, you'll find a new passage opened up at the far left end of the room. Entering inside, you'll find a pair of Yellow Key Doors, and beyond those is a room lined with Backpacks. You'll need the extra stash of ammunition for the room that follows.
V - This large collection of hallways is lined on all sides by elevators. Inside here are a staggering 161 Elevators, and amongst them is the exit elevator that will take you to the level's second-to-last secret level, Level 32: In My Mind. This area is unique in that a lot of the elevators in this area are rigged to take you to the last regular level, or the secret level, but only one elevator will actually be able to be used to take you to the secret level, as the rest are blocked by barrels and other objects. Four of the elevators contain Extra Lives inside them, and the rest are simply inactive. Take the time to find the Extra Lives before you head for the secret level, but be careful; there are exactly eight Fake Hitlers guarding this passage. It would be a shame to come so far, only to be gunned down by Hitler-wannabes.
W - This elevator is the one that will take you to the game's pen-ultimate secret level. If you have trouble finding it, you can identify it because it is at the level's far West end, and is nearby one of the elevators with an Extra Life inside. Of course, if that isn't enough direction for you, this is the exact way to find it from the entrance to this area: from the entrance take the second left, from there, take the second left and immediately turn right and then head right around the corner at the end of the hall, turn right, and then left. There is a right turn on your right, but head to the first elevator beyond it to find the correct elevator that will take you to the secret level.
Speedrun version:
Hidden Text: Show
Speedrunning this level is confounded solely by the small size of itself. Since the halls are bloated with enemies in very tight spaces, you have no choice but to fight every last enemy you encounter, otherwise you'll be pressed on all sized, surrounded, and destroyed utterly and entirely. This speedrun assumes you kill every last enemy and save every POW you encounter.
Start by running out of the starting elevator, and making two left turns and heading through the central door in the starting room. Run straight through the ammunitions stock room and head into the next room. Run around the corner to the right and head into the next room. From here, run around the corner to the left and make a left turn into the next room. Make two right turns and head into the next room. From here, make a right turn and then take a left turn after the table to the right, and turn left again through the next table-blocked passage to the left, and then turn right into the next room. From here, make a left turn and head into the alcove at the far left end of the room.
Collect the Red Key found inside the alcove, and head out through the right of the alcove and head through the door at the other end of the room. Inside the next hallway, head straight through the hallway into the next room. From here, head straight into the alcove, into the fireplace and grab the Yellow Key. Turn around and head back out into the previous hallway. Head straight through the hallway back into the dining room. From here, head into the secret at the other end of the room to the left (on the right side of the passage leading out of the room). Inside the secret, head through the Yellow Key Door and flip the switch inside. Head back out of the secret and head through the door to the right. From here, head back to the starting area (the directions are completely opposite of the ones in the first few steps).
At the start, head through the passage to the left and head through the pair of Yellow Key Doors into the room lined with Backpacks. Head into the large elevator-lined room and follow the directions denoted at point W (Second left, second left, three rights, left, first elevator on right). Head inside the elevator and voila!