Hot off the presses, here comes an innovative new tower defense game from GameInABottle.
In GemCraft, you are a wizard tasked with defending the world from evil hordes of invading monsters—same old story for a tower defense game, but let's face it: we don't play these for the brilliant storytelling, do we? (though the excellent Immortal Defense is a notable exception.)
GemCraft mixes up the classic TD formula with several new tricks. Your defense consists of magically created gems placed atop towers near the path the monsters follow. Various colors of gem offer different types of special attacks—splash damage, poison, armor reduction and so forth. Gems can be combined to produce higher-grade gems which do more damage, and incorporate the attack types of up to three of their component colors (though at reduced potency). You can also place new towers next to the path, or trenches directly on it to slow the monsters down. All of these actions require Mana, which is your primary resource in the game; it regenerates slowly over time and you earn more from killing monsters. 
You can also invest in a "mana pool", which costs a large amount of mana to buy but increases the amount of points and mana you get from kills. Score is determined by how fast you kill everything, and you can get more points by sending in more waves at once, in the same manner as Desktop Tower Defense and some other TD games.
Here's where things get interesting: You can create gems of any grade directly, though higher grade gems are significantly more mana-intensive. However, the gem you get will be of a random color, so if you want a specific gem type you're better off producing a load of low grade gems and combining the ones you want. Combining specific gem colors can get you useful effects, but pure-color gems have stronger special attacks and bonus damage compared to multicolored ones—would you rather have two towers with different pure gems on them, or save space and combine them on top of a single tower? Gems can also be thrown directly on to the path as bombs, which is a nifty way of getting rid of unwanted extras. Another nice touch is that the gems themselves are independent of the towers they're placed on, which means you can position multiple empty towers along the path, create one super-powerful gem and just move it along with the wave of enemies.
The separate levels are tied together in a branching campaign map, and you usually have several different battles available to tackle in whichever order you choose; you can also go back and replay any of your previous battles. Your wizard's level also increases as you win more battles, giving you skill points to spend on various skills that will make you more powerful. These are redistributable at any time, making it easy to try the same battle again with a different skill-set if you're having trouble.
Analysis: On the whole, the game brings a lot of innovation to the tower defense genre, quite an accomplishment considering how crowded it already is. The new gameplay mechanics create a lot of strategic depth, and the game adds replay value by keeping track of your high score for each level, allowing you to go back and replay them for much higher scores once your wizard has leveled up and is more powerful. Each level also has a "gold frame" high score, and achieving it on many battles unlocks bonus maps to play on. If you're a tower defense fan, I foresee this game taking up a lot of your time in the very near future.
