Desktop Tower Defense 1.5
(Note: DTD was yet another game that I played and enjoyed before coming to Kongregate. I suppose, technically speaking, I haven't yet finished it, since I haven't yet earned the Impossible badge, but it's so impossible that I doubt I'm going to any time soon. Anyway, I've certainly played it enough that I feel entirely comfortable reviewing it.)
As of this writing, DTD is currently #2 on Kongregate's highest rated list, and unlike Sonny, I believe it is entirely deserving of that position (if not higher). DTD is the perfect example of what a Flash game should be: easy to learn, but hard to master. The interface is straightforward and easily grasped; the game offers a wide range of difficulty to keep you challenged even as your skill level increases; and the underlying concept is quite a lot of fun.
So, apparently, tower defense games have been around in one form or another for quite some time, although I lived blissfully unaware of that fact until I encountered DTD. (After playing the heck out of DTD, I sampled some other examples of the genre, but none of them was quite as good...but I'm getting ahead of myself.) The basic concept of a tower defense game is very simple: a horde of enemies (called "creeps" in DTD) steadily marches onto your screen, usually with the objective of safely crossing it. You build a variety of defensive systems (the titular towers) in an effort to destroy them before they successfully cross. Some towers will just damage the enemies, but there is of course a wide range of possible effects; some towers do area damage, some will slow or stun enemies, and so forth. Every creep destroyed earns you money, which you can use to upgrade existing towers or build new towers (you can also sell obsolete or misplaced towers); each creep that successfully crosses costs you a life, and should you run out of lives, well, I'll bet you can guess what happens. The creeps also come in a variety of forms: some are faster, some are stronger, some are different colors; you get the idea.
This description, so far, applies to a host of tower defense games. Now, in most tower defense games, the creeps move along a fixed path. This means that there exists an optimal point (or set of optimal points) for you to place your towers alongside this fixed path, and the game reduces to finding these points and then placing as much firepower there as quickly as possible. The big innovation that Desktop Tower Defense introduces is that you create the path -- the creeps begin moving across an empty desk, but as you place towers on the desk, the creeps are forced to move around them. Thus, you can create your own maze and optimize it as you want, and you can even change the maze as it's being built. (Of course, you are never allowed to completely block the creeps' path, but you can create new openings and close off openings that they were heading to.) This adds a whole new dimension of strategy to the game and adds a lot of spice to what can otherwise be a pretty dull concept.
This innovation isn't the only thing setting DTD apart from the host of other tower defense games; it also gets all of the little things right. A lot of tower defense games can drag, but the pacing of DTD is nicely brisk; you'll rarely find yourself with absolutely nothing to do, so it's good news that the interface combines keyboard and mouse to allow you to quickly deploy and upgrade your towers in the heat of battle. (It's also good at presenting information, so you can tell quickly just how powerful a tower or upgrade that you're contemplating is.) DTD also offers an overwhelming host of game modes -- in addition to the basic game play mode, which comes in three different difficulty levels, there's also a bunch of challenges (most of which require you to play under some restrictions, which can make for some quite interesting games) and "fun" modes (these are somewhat silly modes where the rules of the game have been altered somewhat for unpredictable results). The result is enough to keep you challenged from when you're just starting out all the way up until you're an excellent player.
I actually cut my teeth on version 1.2, which is also available on Kongregate, but the new version, 1.5, is by far the more popular. 1.5 features some different modes, a few new creep types, and a couple new tower types (which I haven't really gotten the hang of yet), making it overall somewhat more complicated but still very easy to learn and handle. The presentation is not bad. The graphics are clean and simple, and the sounds, while a little basic, are well-chosen to be fun rather than annoying when hundreds of them are going off at a time.
Overall, DTD is a tremendously entertaining game. If you have fifteen minutes of spare time, sit down and give it a whirl, and you should have a lot of fun.