This is the third level of my game so far. This is overall how it looks. As this is a GZDoom-based game, this is how it looks in the level editor but this is otherwise 3D. Anyways, it all starts in this area in the curved room that comes from the previous level. I've highlighted the areas the player visits normally in the picture below.
The first part of the level is very linear, just a hallway, but the player has to find a cyan key in the blue room they enter by moving down. After that small key hunt, they can open the door at the end of the hallway then jump down the stairwell to reach another hallway that leads outside to the city streets.
In both of these areas, the player can approach them differently; For example, there's a big secret path (the big winding path in the picture above) which the player can find by entering the vent seen overhead in the room with the key. When the player goes outside to the city streets, they can perform the wall jump to get up to the streets faster or just find another door around the building that takes them up to city street level. If they do this, there's a single turret ready to fire at them in that hallway.
Here's a 3D view of the hallways with that turret visible; It's harder to talk about 3D level design than 2D.
The city streets are really not interesting to talk about as most of this level hasn't been made that detailed, but here's a 3D picture of a part where the player can break through certain windows on these buildings then use springs to bounce to other parts of this area. (All of those circles are visual representations of how far out each light's buzzing sound and pop-in distance is. To keep glitches from happening and possibly improve performance, I make dynamic lights turn off at further distances.)
This area isn't fleshed out yet so there isn't much level design to talk about my reasoning behind it other than the springs and two buildings in that picture.
Okay, except for this setpiece that happens in the building on the right; There's a Powerup Coin in the "hallway" there, but if the player tries to grab it, a Troll enemy jumps through the window behind it and hurts the player if they don't react in time.
I feel I'm making this too long so to keep it short, let's just look at the bowling alley you'll reach if you go down enough streets.
The player enters here from the top and can knock pins down for coins. The main thing to do here is to find a switch hidden behind the front desk then go behind the lanes and use the vent back there to reach a secret area that is blocked off by force fields if the player comes from the right. The player gets a Powerup Coin here but has to avoid the acid which slowly damages you on contact. Okay, that's enough, so here's a final picture of that room with the two paths that branch off from this room.