Downtown was perhaps my least favorite of all the Doom 2 maps, at least until my most recent play-through recorded below.
The hub\mini-quest, free-form nature of map 12: The Factory is mimicked here also, as the overworld is a go-anywhere outside environment, punctuated by various structures and secret side-doors, but the map really is quite linear. There are only perhaps 2 or 3 structures you can enter at will, whereas the bulk of your progress through this level is usually thanks to an unlocked door brought about by a previous switch, brought about by a key, brought about by killing a specific creature. About 80% of your progress here has to happen in a particular order, leaving the top 20% as trigger-free choke points and killboxes.
This map, like most Doom and Doom 2 maps, features its fair share of traps and secrets, but that isn't what gave me the most resistance.
The sheer volume of monsters and my severe lack of ammo, health and armor made this one a particularly tense play. To illustrate this a bit, I paused the game about 20 minutes in so that I could eat a chicken pot pie I had warming in the oven. Something went wrong with FRAPS and when I resumed recording, FRAPS overwrote my initial 20 mins. Even with 20 minutes shaved off my playthrough, this is a 37 minute video. That should indicate how much of a beast Downtown was to finish on Ultra-Violence difficulty.