I managed to beat original Dungeon Keeper campaign and it was a lot of fun. However Deeper Dungeons expansion is IMO unfairly hard. It definitely is way over what a "normal" gamer can digest.
I beat both the base campaign and Deeper Dungeons one or two years ago, and for some reason I recall having much more difficulties and frustration during the base campaign, than the Deeper Dungeons missions.
I can't put my finger on what it was, but I seemed to get stuck much more often in the base campaign missions. Maybe they had less "endless gold" blocks (so you could actually run out of gold completely during a level), or I just learned to play it better, or somesuch.
Overall I enjoyed the game, but sometimes waiting for your minions to train themselves to level 10 bored me to death, and the game mechanism always seemed odd and obscure to me, ie. what you are supposed to do with different rooms, which enemies needed what rooms, which enemies didn't get along, did it matter if your rooms have walls or not (other than blocking enemies from coming in), what was the reason that sometimes some minions got angry and would leave your realm (sometimes I knew it was because I ran out of gold, but quite often I didn't really understand what was the thing annoying them, and what could I possibly do about it), etc... None of it really seemed that logical, it just was the way it is, and you had to memorize all of it.
Reading your reply, I am actually amazed that you defeated the expansion. Are we really talking about the same expansion, Deeper Dungeons? :)
Most of answers to your questions can be found in the manual. Actually it seems to me that the game sort of assumes that you have read it and you know what you are doing. Every kind of creature is explained there, as well as all the rooms, spells, etc.
What makes the expansion a lot harder are some artificial limitations that the game puts on the player, like: you cannot have more than 20 (or sometimes 10, and sometimes even less) creatures in your dungeon, while enemies can have up to 30. Or you cannot build certain types of rooms, like Prison or Scavenging Room, and so on. This makes the game fricking hard. On second level in the expansion player finds himself with 20 creatures and going against two other dungeon keepers, each having 30 creatures. Things like that make it almost damn impossible to enjoy and very hard to finish. Not to mention later levels. I read parts of the walkthrough. There are levels where you control one(!) Spider and have to beat enemy armies and heroes with it. It requires incredible luck and skill in drag-and-dropping creatures, placing traps and whatever. And this is in turn almost impossible to me, because the game runs like a piece of crap. During the fights I have a terrible slowdowns, even KeeperFX doesn't solve it entirely (although it helps a little). I mouse-click on something and the click is not register, I try to cast a spell and it doesn't work, and my creatures are dying. There are lags and UI is unresponsive. It is virtually not possible to cast any spells during the combat. This is very frustrating. All these kind of things ruin the game. The base game is easy enough to somehow live with them and still enjoy it, but expansion requires perfection from the player.