Yep, entirely made with GMS2 - no fancy extensions or anything. One of my goals with this project was to make a type of game that people generally don't associate as something you'd make with GameMaker. Early on, I toyed around with the idea of making a racing game, but then I cranked out a simple maze generator to help test out how I was going to handle collisions and navigating the maze felt really good so I shifted focus to an FPS instead.
Thanks for noticing the visual health indicator things on the enemies. I didn't think it felt good to just have the enemies sponge a bunch of damage with no sign of progress, but I didn't want to just have floating health bars or numbers either- so, having a bunch of light bulbs or something floating around the enemies seemed like a nice way to be a little more subtle and on theme about it.
The letters on the walls represent cardinal directions and were originally for testing so that I could make sure my level generation algorithms were building the vertex buffers and setting the uv map coordinates correctly. As for the wall colors, you have to light up at least 50% of the maze green to activate the exit and when an enemy turns a green set of walls magenta, that does deduct from your active percentage of the maze. I don't think a lot of people have run into this, but if you get an enemy or two to chase you across the maze, they can actually take over big chunks of the maze real quick and make it difficult to get control of that 50% it takes to activate the exit. In a future revision, I'd really like to add something to the GUI that shows how much of the level is under player/enemy/neutral control to make that a little more clear.
Thanks for playing!