If you're looking for themes for a 'level' type construction, I've got a few.
One way in, one way out:
Design the progression so that once you've entered the stage, your only way out is to proceed through the entire course. A 1z fall would be the simplest method of a 1-way entrance, just make sure the floor below is of a low density material to minimize damage. As for an exit, a lever at the end which lowers a drawbridge and unblocks an exit would be a good example.
Backtrack Hell:
You make a grand treasury containing ALL OF THE NAZI DWARVEN GOLD, and seal it behind a corridor of drawbridges. The levers to unlock these drawbridges and unveil the lewt are scattered as far away as possible from each other throughout the fortress. You can design the layout so you can pull the levers in any order, or in a way so certain areas and the levers within are inaccessible until certain conditions ar emet, such as draining the magma out of a corridor. Furthermore, monsters are released with every successive level, because that's how real backtrack-a-thons break the monotony.
If keys and door locks are ever implemented, can substitute those in and save yourself a lot of mechanism dickery. Also, if you choose to neglect setting the area as a lair, those keys would be scattered around the site and give it a real roguelike feel.
Boss Rush:
A linear progression where you go from room to room and fight the enemies within to progress. For best results, seal the mobs in their room somehow so a)they don't all gather in the first room and bum rush you, and b) you have to unlock the exit door to progress. While nothing would stop you from unlocking and proceeding to the next room without finishing off the enemies in the current room, it is inadvisable as you'd have to fight two rooms worth of monsters at once.
The Water Level:
For the masochist who actually enjoyed that one dungeon in that one Zelda game or the asshole who loves to troll their friends when designing a level for them. Most player forts end up with a flooded section at one point and this area prepares for it. The objective here can be to either make the adventurer swim their way through, or make it so progression is impossible to air-breathers until the water is removed. he biggest drawback in designing this area will be planning for multiple z-levels the massive water-flow induced lag.
The Fire Level:
Player forts should have one of these even if they aren't intended for exploration. Narrow walkways suspended over pools of liquid hot magma, where one false dodge can spell fiery death. Otherwise impassable magma pools could be circumvented by diverting flow from a water flooded area to create a bridge of obsidian.
Transitions:
It's important not to just shove everything you have planned into a giant conglomerate and say "Here you go"(unless that's the actual plan). You need pacing, a divider between one area and a next, especially if you're going with themed zones like fire and water. Something as simple as a narrow hallway or long staircase will suffice. It also gives the player a bit of breathing room, a chance to calm down and prepare for the next phase. As such transitional areas are perfect spots for safe rooms, places you can lock yourself in and recuperate in.
These examples are not exclusive from each other and could be combined to create a grand old adventure.
If you find yourself with a mental block, just watch someone else play a video game on Youtube(because ain't nobody got time fo doing dat demselves). If the ramblings of a nasally let's-player don't manage to distract you, pay attention to the layout of the stage, dungeon, or what-have-you. Take note of little details, such as the transitions between different areas and the positioning of landmarks. Part of what makes classic games so great is how their levels were planned, and they can have a lot to teach an observant player. Food for thought.