Defense Grid the Awakening

Developed by Hidden Path Entertainment and released for the PC in December 2008 (and Xbox Live Arcade in September 2009), Defense Grid: The Awakening is a Tower Defense game that's been hailed as one of the best in the genre.

Over a thousand years ago, an Alien Invasion on your planet was beaten back. Now, the aliens are invading again, and your world is in peril. They've besieged your city, cutting you off from your home. In the nearby ruins of an old military base, you find a dusty terminal and reawaken an ancient Artificial Intelligence--who, in reality, is the very general of the previous war! After the war his brain was uploaded into the planetary defense grid, in case the aliens ever returned.

With the help of the General, you've jumped into the command chair to re-awaken the grid and Save the World.

Defense Grid provides examples of:
 * Airborne Mook: Darts and Mantas. Both of which do not take the usual route which the other mooks do, and instead travel another fixed path.
 * Alternate Reality Game: Is one of the 13 indie games that form the bulk of the material of Valve's "PotatoFoolsDay" Portal 2 ARG.
 * Boring but Practical: Once they're fully upgraded, Gun Towers are one of the best damage-dealing towers available. They're also the cheapest tower to buy and upgrade.
 * Brain Uploading: The General who stopped the previous invasion is now the AI running the defense grid. He often talks about things he would do if he was still in his body. Mostly eating raspberries.
 * Canon Immigrant: Guess who stars in the expansion Defense Grid: You Monster!? Hint: Not Chell.
 * Chain Lightning: The Tesla tower can chain up to three hits.
 * Cloudcuckoolander: Spending over a thousand years uploaded inside a defense computer network has left the General a little... odd, but he realizes how he's acting and resolves to stay focused on repelling the invasion.
 * Crossover: The You Monster expansion crosses over with the Portal series.
 * Deflector Shields: The Bulwark uses these to protect itself. The Spire's can protect other aliens. The already unfair Crasher is given these to make The Juggernaut. Gun and Cannon towers are Super Effective against them.
 * Dungeon Bypass: Later levels have you build The Maze yourself using towers. Flying enemies go right over it, and instantly leave the map when they reach your power cores.
 * Elite Mook: The description for the Rhino states that it is the basic elite unit of the alien army.
 * Excuse Plot: A well-done example of one. It's a rare thing to see a Tower Defense game with Character Development.
 * Exposition Fairy: The General himself.
 * Fragile Speedster: Racers can only take about as much damage as a regular walker, but they are a lot faster.
 * Frickin' Laser Beams: Laser towers, which are good against the two fast enemies, the Racer and the Rumbler.
 * Gatling Good: The level 2 gun tower swaps out the chain gun with a tri-barreled gatling gun. The level 3 gun tower adds a second one.
 * The Goomba: Training drones in the first stage are very weak and can only carry one core. That's the only time you will fight them, though.
 * Hey, It's That Voice!: The General assisting you is Captain Qwark and Sentinel. Worried yet?
 * Kill It with Fire: Inferno and Meteor towers are strong against Swarmers.
 * Kill Sat: The orbital laser obliterates anything, but is slow to charge.
 * Lightning Bruiser: Rumblers. Pretty much a King Mook version of a Racer, these guys have quite a lot of health, are extremely fast, and they can carry away three cores at a time! In terms of boss standards though, they are Fragile Speedsters, as they can only take about fourth of the damage of a Crasher.
 * Lightning Gun
 * Meaningful Name: Almost all the Mooks.
 * Swarmers always appear in swarms, unless they're periodically released from a Seeker.
 * Rhinos are heavily armoured and can take more damage, similar to how the real life rhinoceros has a thick armour.
 * Racers are very fast enemies.
 * Darts have a dart-like appearance, and they can fly like one too.
 * Mantas looks kind of like manta rays, and also fly through the air in the same fashion the manta ray glides underwater.
 * A bulwark is a protection against external attacks. Bulwarks have Deflector Shields that protects them from damage.
 * A spire in real life is a pointed structure. Spires in this game are shielded aliens that have a pointed head.
 * Lurkers have the ability of stealth protection and can "lurk" away from your towers.
 * Decoys give stealth to units around it but do not have stealth themselves. Thus, they act as a decoy and attract attacks from your towers towards it, instead of the enemies it gives stealth to.
 * Turtles have heavily armored shells, like a real life turtle.
 * The Crasher tends to inexorably crash its way straight through whatever you throw at it, while its shielded counterpart The Juggernaut is, well, nigh unstoppable unless you have strong weapons.
 * Meta Powerup: Command Towers are these--they don't add any extra power for your towers, but they will reveal Stealth units and also allow you to gain more resources from any enemies killed in its range.
 * Mighty Glacier: Crashers and Juggernauts. These guys can take a real beating and can make off with three power cores at once, but due to their size and weight, they are slooooow (in fact, they are the slowest of all the aliens). Juggernauts are Crashers with Deflector Shields that make up half their health.
 * Mook Maker: Seekers periodically open a portal to the enemy aliens' home planet, allowing (usually) smaller but weaker aliens to join the fray. In the final campaign level, there are Seekers that will generate nigh-endless waves of Crashers and Rumblers if they aren't dealt with.
 * Turtles are heavily armored boss enemies that carry weaker Mooks inside, which are released when the Turtle is defeated. In later levels, they carry Rumblers!
 * Ridiculously-Fast Construction: Towers rise up almost fully assembled from hatches in the ground.
 * Shell-Shocked Veteran: The General is often overcome with the tragedies he's experienced a thousand years ago. At one point he briefly thinks he's still fighting the previous war.
 * Shock and Awe: The Tesla tower. Its attack is stronger if given time to charge up.
 * Turtle Power: One of the enemies is actually called the "Turtle". It has a lot of health, and just like a turtle, contains stuff inside its hard shell.
 * Who Wants to Live Forever?: The General.
 * You Require More Vespene Gas: Towers are built and upgraded with a single Gold-type resource from killing aliens which doubles as your score, along with the remaining cores and total sell value of your towers at the end of the level. The game encourages minimal use of resources through interest.
 * Zerg Rush: Swarmers. Weak health, but damn, they come in large swarms!