Dungeon Defender is an interesting game -- it is an attempt to add some RPG elements into the traditional tower defense structure. While the effort is not entirely successful, there are some interesting ideas in here I'm glad the game designer took a look at.
The basic concept behind the game should be familiar to anyone who's tried a tower defense game. Waves of enemies come in and attempt to reach your base; your job is to build structures which will destroy them before they do so. So far, entirely normal. However, in this game, your structures aren't just static buildings which fire at the enemy. Instead, they are lairs which spawn creatures. These creatures then go out to attack the enemy (if they are melee) or fire weapons or spells (if they are ranged units); the enemy similarly has different types of units which will engage you differently. Your creatures have a finite amount of health just like the enemy's, and if the enemy happens to kill them they will be free to proceed unimpeded. However, your creatures will respawn after a short amount of time from their lairs. The enemy can also destroy your lairs simply by walking over them, but it won't go out of its way to destroy your lairs, so if you don't place them along the direct path to your base they'll be safe. (Of course, it may be harder to successfully block the enemy if you do that!) Note that you play the role of evil in this game, so your creatures are of the typically evil type (goblins, vampires, hydras, etc.). You can also augment your defenses with traps, which do various nasty things to the enemies (though the number you can place is strictly limited, so you can't just fill the map with traps) and support buildings, which increase the effectiveness of your lairs. There are also some maps that contain neutral lairs, which will fight any creature (yours or the enemy's) that stray nearby. The map begins with some paths running through the dirt from the enemy spawn points towards your base; you can dig out more dirt if you want more space to build lairs, but you can never fill space back in. Some dirt also contains precious metals that give you more money; these are usually cleverly placed so that digging out the most valuable deposits will also give the enemy a shorter route to your base, so you have to balance your monetary needs with your defensive ones.
There are also a few other RPG-like additions to the game. In addition to the lairs, you also have an avatar, which you control directly, and can be chosen from one of three different classes. The avatar is a powerful fighting unit (and hence is useful to throw in at the point where your defenses are weakest), but he can also be killed, in which case he will respawn at your base after a while. The avatar gains experience, and as he reaches higher levels, you can build more types of creatures and defenses. Similarly, unlike normal tower defense games where you increase the power of your towers by upgrading them, in Dungeon Defender your lairs gain more experience as they win battles and the units that they contain thus become tougher. Units also have their own strength, dexterity, and magic stats, and defeating certain enemies will give you items which can be equipped on your avatar to improve his fighting stats.
If you've made it this far through the block of text, you're probably beginning to get an idea of the first problem with the game: there's just too much crammed in. For instance, are you ever you ever going to look at the items on your avatar? Well, there is (unfortunately) a fair amount of idle time in the game, but even during that, you're probably not going to be interested in comparing the different kinds of breastplates that you might have picked up. Similarly, it's generally too much information to have such detailed stats on each creature -- all you really need to know is how generally powerful and fast they are, not the minute little details of their strength and magic resistance. Having to micromanage, for instance, the distance melee units should travel to engage the enemy also becomes quickly tedious.
The much more serious problem, though, is that it's frequently impossible to tell what's going on. As usual, units have little health bars over their heads. However, as I mentioned earlier, a lot of the units are melee units. That means that oftentimes, you'll get ten or so units in an extremely small space, and it's simply impossible to tell what's going on, and then eventually some units begin to emerge, and hopefully they're yours, but it's impossible to tell why or really what happened. This makes it extremely difficult to get feedback -- the lifeblood of making a tower defense game (at least one which isn't a total pushover) is to give the user the opportunity to see what strategies work and what strategies don't. With this, though, it's impossible to tell which units are pulling their weight and which ones are completely ineffective, so it's very difficult to tell why a given strategy isn't working and what you might want to do to fix it. I find this personally extremely frustrating, and it's also why I resorted to using a walkthrough for four of the last five levels, simply because I found it so un-entertaining to try and fail without really being able to tell what I was doing wrong.
The graphics are pretty tiny, since there's a lot going on the screen (which only exacerbates the above-mentioned problem), and there aren't any sound effects, only music, which, despite being on an exceptionally short loop, isn't too bad (and there's more than one tune, so at least it doesn't become incredibly annoyingly repetitive).
In the end, this was a game which I found intriguing at the outset, but which really became a slog as it went on, and I was glad to get it finished. I like some of the ideas in the game, but it really needs more polishing to become a great game.