Click here for a very well done gameplay description youtube vid (Please ignore his Dungeon Keeper comments.)
Alright! So here it goes.
I wouldn't say that this is my favorite game/series ever, but I do think that it is way too overlooked and discriminated against. I went over the reasons for that already, but I really think that taken on its own merits, these games are actually quite unique and with all their inidividual elements coming together, very satisfactory to play.
What it all comes down to,
is basically a kind of reverse tower defense. Not only in theme (since you are the badboy slaying the heroes), but also in mechanics. Your placed "towers" exist only to satisfy the needs of the ones you need to kill. Still with me? Alright, that is because the core concept revolves around killing "satisfied" heroes. The more needs of a hero you satisfied, the more "soul energy" he will give you after you killed him, which in turn can be used to use some Dungeon spells or buy items for your dungeon to get even MORE soul energy from those heroes.
Thats more or less how the game works. Heroes come, you satisfy their needs, kill them and use the money and soul energy gained to improve your dungeon. I need to admit that there really could be a bit more variety to that. Like actual (and devious) traps to lure your heroes into etc. However, they still managed to make this a rather fun affair due to some aspects that work together rather splendidly.
First of all, there is an overall levellingsystem of your hero, giving you steady progress and new skills throughout the campaign, that make new campaign maps feel different just by the fact that you now have different skills to use than in the map before:
Additionally, linked to that, there are several integral achievements on each campaign map that will give you skill points if you complete them. Those vary quite a bit between the levels, making the levels feel even more different. After all you will play differently if you need to save up to a certain amount of gold, if you are to never reach a certain amount of gold, or if you are supposed to abstain from using something you would normally use.
These may sound like small things, but actually go a great way of ensuring you never quite feel like doing the exact same thing over and over again. These games are by no means a revelation or the hardcore dungeon simulation people were hoping them to be, but they are definitely fun (after the initial tutorial area) and have a well thought out progression. Lots of diversity and content too, I'd suggest giving the first one a shot. You could do far worse with your 5 bucks. Also: watch the vid in the beginning of the gameplay section, it does a very good job of conveying the game mechanics.