Generators
While some enemies already exist in the level, the majority appear from generators. These look like piles of bones or large blocks with skulls on the top. Enemies pour nonstop out of them.
Unlike monsters, generators take three hits of anything (including magic) to destroy regardless of what your character's attack or magic levels are. Each time it is hit it crumbles a little, and the monsters that spawn from it will be similarly weakened and take fewer hits to kill (although unlike the generator the monsters are easier to kill if your attributes are higher).
After three hits the generator is gone. Unless you're aiming for a big score, taking out generators the moment they appear onscreen is a good idea. Generators do not start producing enemies until they first appear onscreen, and production ceases if they are far enough offscreen.
Enemies
As a general rule, enemies on their own are not a problem, but be careful when going into places where swarms of enemies could trap you indefinitely.
Ghosts
Ghosts die upon contact, taking a sliver of your health with them. Grey ghosts take so little health they are little more than a distraction.
Grunts
These enemies melee you. If you keep out of range and shoot them or blast them with magic they can't even touch you.
Demons
These shoot fireballs. The weaker the demon the less damaging the fireball. These are the most dangerous standard enemy.
Sorcerers
These enemies constantly blink in and out of visibility. They cannot be damaged when invisible, and the varying visibility can make it hard to see how large a group of them is.
Lobbers
These enemies can throw rocks quite a distance, even over walls. They have no melee attack however, so once you get within about three tiles of them they will start running.
Death
This ninja-like creature chases you and drains up to 200 HP before vanishing. You can gain a good deal of points by shooting them, so if you are behind a diagonal wall you can easily shoot through the gap to boost your score. The only thing that can harm them, however, are potions.
Thieves
These annoying enemies steal your stuff and run off. Due to their immense speed and erratic movement it can be difficult to even catch them let alone kill them before they touch you.