Instant Dungeon! is a game with a simple concept and a clean, polished execution. It drops you into a procedurally-generated dungeon populated with items, monsters, and treasure, and your job is to navigate the dungeon, use the items to avoid or kill the monsters, get the treasure, find the key to the next level, and get through the door. It's even simpler than it sounds: the dungeon halls are only wide enough for one, and one touch from a monster kills you, so you need to find ways to circumnavigate the creature or kill it. You carry one item at a time, use it with one key, and every item is single use. In the end, you have three functions: move (one of four directions), use item, and pause. Simple. Not necessarily easy.
As you play, the game scales up complexity. Dungeons become larger, more intricate mazes, with a greater number of monsters. The items scale up in complexity too, and require increasing tactical awareness to use well: the dagger kills the first monster it hits. The axe kills every monster in a straight line until it hits a wall. The fireball is like the axe, but fires in two directions. The shield protects you from one hit, but you have to use it deliberately. The helmet you can put on, and it will protect you without action. And so on.
Complexity doesn't always mean difficulty, and it wouldn't here except for one additional factor: the darkness. As you go deeper, the darkness becomes more oppressive, until you can't see any more than what's in nearby line-of-sight from your character. This ramps up the difficulty more than the size of the mazes or the monsters, because you become far more likely to turn a corner into an enemy, or to get boxed in by two monsters you can't see.
Speaking of complexity, I'm not sure the procedural generation adds much to the game. The levels are rather similar anyway. It feels rather like the gameplay/decision-making loop is too small for the fact that the level is different than it was last time--or even than the last level--to make much of a difference.
Every fifth level is a boss level, with a unique monster whose attack pattern you must figure out to get the treasure. The boss levels are authored and break up the routine of mazes and monsters.
Multiple game modes don't add much to the game. As simple as the game is, it's hard to see how they could. Instead of getting the most treasure, the other modes require you to kill a certain number of specific monsters, or to rescue people from the dungeon. This last is the only mode I've seen where you backtrack through dungeon levels you've already been through, and also the only time I've seen a bug in the game. (The third captive was displace to be off-map, so I could see the "Help" speech bubble but not reach the captive, making the game unwinnable.)
Instant Dungeon! has a nice collection of character sprites for you to choose from, of all genders and types, and it defaults to a random selection. The captives are likewise drawn from this pool of sprites, and not always any particular gender. I thought that was nice, from a feminist point of view.