Defender

Brief guide
Defender is a quite simple tower defense game in which you must buy, place, and upgrade units in order to stop monsters from reaching the portal at the end of a path. Monsters will always follow the path, and units can only be placed on the walls. Units vary in strength, speed and of course cost, while enemies vary in speed and weaknesses (for example, bats are weak to ranged attacks but immune to traps.) Units are bought for gold, which is gained by killing monsters. Once a unit has been placed, it can't be moved, although you can sell it for half the price you bought it for.
There are four different categories of units: Rangers, Psychics, Mages, and traps. There are four units of each type, but you will have to upgrade categories to unlock more units of that type. Click on a category at the bottom of your screen to upgrade it.
Traps work like other categories, except you place them on the path instead of the wall. They will damage enemies who walk over them (unless, of course, the enemy flies over them, for example birds and bats).
Once a unit has gained enough kills it will show an upgrade symbol. Click on a unit with the upgrade symbol to level it up in exchange for some gold.
Even if this doesn't sound simple, don't worry. It's easy to get into!
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NEW (March 3rd 2013):
I have just put this game on my work portfolio. To anyone who might be reading this as a result, just a quick informal warning that the art is terrible and and the balancing is way off. But it's still awesome.

NEW (April 10th 2009):
A minor but important update, as well as changing the colours of Defender's interface very slightly, I have been tracking down a very discreet loop in the code, which caused the game to begin to lag more and more immensely after about 20 minutes of playing. I don't know if it's been fixed; I'm going to have to do some online testing with other people first... So, let's get hosting, people!
Oh, yes, and there was a small mistake where the Void trap's price was shown to be 520g, but it actually cost 540g. It has now been set to 520g, and my apologies for not noticing sooner.