GemCraft is, at its base, Just Another Tower Defense game, but it's so pretty and well-executed that it manages to remain interesting for longer than you might expect.
The basic layout of GemCraft should be familiar to anyone who's ever played a tower defense game. There's a path, with plenty of twists and turns, and enemies walk along the path. The object is to destroy them before they reach the end of the path by building various defensive structures. Now, one interesting twist to GemCraft is that these structures aren't fixed. Well, you build towers along the sides of the path, and those are stationary; but, by themselves, the towers don't do anything. You create gems and put those in the towers, and then they can fire on enemies. You can also throw gems as bombs directly as enemies; I tend to prefer using my gems towers, but there are apparently some people who use strategies which involve almost exclusively throwing gems (which makes it quite a different kind of game, I suppose).
As befits its fantasy nature, there's no cash in the game; rather, your currency is mana. You gain mana both over time and by killing enemies; summoning gems is, naturally, the main way of spending mana, though you can also build more towers (you generally start out with a few, but you may wish to place your towers more strategically) or moats along the path to slow down enemies. One other unusual feature of GemCraft is that, as you might expect, various types of gems have various special abilities (eight in all), which correspond to eight possible gem colors, but when you create a gem, you don't get to pick the color -- it's randomly chosen. This can be a source of great frustration. (However, in a typical map, only a subset of the eight possible colors is available -- all eight appear only in the epic boss levels.) You can also combine gems to create more powerful gems -- you can create gems of levels 1 through 6 (with ever-increasing costs, naturally), but you can also combine two level n gems to create a level n+1 gem. Combining gems of two different colors yields a dual gem, which has some of the special powers of both, but these tend to be slightly weaker than a pure gem of the same grade. (You can similarly create tricolor gems or gems with even more combinations, but these are even more strongly disfavored.) Trying to combine two gems of different grades will just yield another gem with the higher grade of the two combined, so there's usually not much point.
As in many tower defense games, though, the real strategy comes in managing your money supply. You have one spell, Mana Pool, which increases your mana total and mana gain rate; thus, it is essentially equivalent to interest in a normal money-based game. Not casting enough Mana Pools is the prime cause of defeat for beginning players -- if you don't do it enough in the early part of a map, you'll never have enough mana to build the more powerful gems you'll need in the later part of the map. Conversely, if you do well enough in the beginning, often you'll find yourself swimming in mana by the end of the map, so that the later waves are a cakewalk. If enemies reach the end of the path, some amount of mana is deducted and they return to the beginning; if you should run out, you lose that map. This usually happens only due to carelessness, or due to the epic bosses.
The game is huge -- there are a total of 48 maps, including 5 epic boss levels and 8 hidden levels, which are revealed by getting a "glowing frame" on other maps (obtained by attaining a sufficiently high score). The overall layout is not entirely linear, so you don't have to play all of the levels, although you of course have to beat the epic boss levels. Being a completionist (and since you need to beat them all to get the hidden levels, which are required for the last badge), I naturally played them all anyway, which took a fair amount of time. Each individual map has somewhere between 8 and 50 waves (following the normal pattern of normal creatures interspersed with the occasional boss wave), with typical maps probably being somewhere around 30. Fortunately, each wave is relatively short; 20 creatures is a pretty large wave, and even non-boss waves can have as few as 3 creatures, so an individual map goes by pretty quickly. The game does a good job of avoiding the dead time which plagues games of this genre; you can quickly send new waves if you've already defeated the existing one, and you can also speed up time if things are going slowly in general, so you don't have that much time sitting around twiddling your thumbs.
As you clear maps, your wizard gains experience, which can in turn be used to improve skills which help various aspects of your gemcraftery. This brings me to the first complaint about the game: the difficulty is very uneven. The first few levels are very easy, and you don't really need to develop much strategy or learn much about careful play, and then you hit the first epic boss, which is quite difficult. You'll need to become much more proficient at carefully managing your mana (and much more aggressive in using Mana Pool) in order to beat it. (Looking at the comments, I'm far from the only person who hit a difficulty jump at the first epic boss.) Then, once you've developed your proficiency, the game goes back to being pretty easy (although no longer completely trivial), until your wizard accumulates enough experience that you can reach the really high-level skills, at which point the game becomes embarassingly easy. After I reached that point, the rest of the game was more time-consuming and not challenging at all, so I wish the designers had found a way to alleviate this boredom somewhat (possibly by not making the high-level skills so powerful to begin with). My second complaint is somewhat more trivial: with eight colors, it's of course going to be hard to keep them all distinct, but still, that's no excuse for having two of the colors be "lime" and "green", which are nearly indistinguishable to my eye. The blue and purple also look awfully similar, and it's very easy to get confused in the heat of battle.
The presentation of the game is absolutely gorgeous -- the graphics are excellent, and the sound is also well-done. (There is no background music, however.) But what really makes this game stand out is the attention to the interface -- buttons make a little click when you highlight them, tabs slide out, information is always easily accessible; it's very well put together and makes it feel like a much more professional game.
Overall, this game felt a little longer than it needed to be (especially since the final ending was a little anticlimactic), but it's definitely a game that's worth playing. Even though it's an old formula, this is so expertly executed that you can have a fun time playing it.