Specifically, how would I use triggers to, say, spawn more monsters or a pickup or open a gate after a certain wave of monsters are defeated? I know of the NoMon trigger, but I can't get it to work correctly.

You need to use the nomon trigger in combination with setting a time in the timeline at the bottom. At least it worked for me, still trying to figure everything out :)

Move the timeline cursor to a point in time after the wave of monsters were spawned, then drop your new monsters or pickups in the room and set these to the nomon trigger.

NOMON makes the object appear when there are no more monsters.

A trick I use is to have a single "tripwire" object, which are simply two voxels separated by an empty space within the same object. I group that object with the ones i want to trigger with (Shift + G when you select them all), set their triggers to GROUP, then set the trigger of the tripwire to whatever suits you.

EDIT: Check my WAHNTHAC map to see two instances of the trick.

I'll take a took at it, but it doesn't seem to be showing up on the BBS list. I suppose I'll wait. Until then, Waitwaht, could you explain further on the tripwire? How does it work?

And thanks, Chris - I had no idea how to use the timeline.