Actually, monsters will spawn in some areas in ultima 2, though they are a little limited. In the ice caverns in a few spots with ice worms, they will spawn if you sleep, and in at least one yeti spot in level 2 near the northern section, near the entrance to the level leading from the dimension's start, a yeti will spawn. The trouble is as far as I've seen it requires sleep, so you kill whatever spawned, sleep, new monsters, sleep, new monsters to slay, sleep, and of course that requires loads of food, so either you need to be constantly fishing or using tons of create food spells.

I know if the skeleton generator you speak of, but in that mage academy there is also a mongbat generator. On the very first test, if you open the door to the middle room, there is a hidden trigger space just at the entrance to the room from the short hallway past the opened portcullis that spawns a mongbat.

I've spawned dozens and dozens to kill and get skill points, but I'm actually afraid of some entity limit creating troubles after killing so many and having so many blood splatters, however I do not know if there IS a limit or if so what it is.

Another spot you can sleep and kill spawned monsters is actually at the very blackrock gem, however I believe the enemy pool is changed during the course of the game as you unlock more worlds and enter them, so tougher monsters will spawn if I am correct as you sleep.