Anyone mention Anomaly: Warzone Earth yet?
It's reverse TD, looks interesting and is related to the topic ;)

Dude, it looks like you really send your units into the meat grinder in that game. Interesting concept, I'll have to look into it:)

+1

Blizzard Tower Defense is amazing and challenging.

Harvest: Massive Encounter

and this thread from the steam forum .

The best TD games that I've seen, by far, are user maps for Starcraft, Warcraft 3, and Starcraft 2.

Nothing commercial comes close, IMO.

@ Orc

While Dungeon Defenders has been delayed on PC and console you can play it on iphone and some pad thing or something... I dunno these fancy schmancy devices.

I'm more of a Linux guy so I have an EVO, but thanks for the tip:)

Seconded.

Immortal Defense (pay-what-you-want, demo available, PC only for now) is a tower defense game with a story on the level of Planescape: Torment. Highly acclaimed, GameTunnel's Strategy of the Year (2007), considered "better than Portal" by a prominent reviewer. Why?

1. Short timed levels means no scheduling concerns, unlike in conventional TDs ("I have an appointment in half an hour - what if I start winning?").
2. Again, short levels lend to the realization of tactical depth, because trying new tactics doesn't waste too much time.
3. Adjustable difficulty makes the game playable to completion by players of, ahem, varied tactical aptitude.
4. A very fair scoring scheme encourages good performance but doesn't create a spiral of doom problem.
5. Value per dollar. 100 levels in the story, 50 bonus levels, a level editor.
6. Story, visuals, music - awesome all around. The story is mainly told in mission briefings, but level shape, tower availability and obstacles also matter.
7. Meta-story and use of medium: beyond compare, unbelievably perfect.

Videogame haters have a name for use of medium - "why haven't you used a more respectable form for your story?" The most common answers are that the game is meant to be lost a number of times to give an appreciation for the difficulties the characters face, or that the story is meant to be experienced from multiple viewpoints to see how it all fits together, or that it is meant to be explored in multiple directions, or that it's very important that the player is given control over the character's choices. Now, the story in Immortal Defense is linear, and none of those standard reasons really apply (except maybe the final one, if you consider "stop playing" a character choice). Despite that, it's the absolute pinnacle of the use of medium, the game that can shut up Roger Ebert and make Yahtzee put forks into his eyes, a true masterpiece and a must-play.

(This is also why, despite the high praise from players, it hadn't gone viral: the awesomeness that is contingent on medium just gets lost in translation to words and screenshots and gameplay videos, and "must-play" is all too often an annoying double-pronged bad-logic trap for critics. But not in this case: sometimes awesomesauce is just awesomesauce.)

FAQ
1. No DRM at all. Not even a game key.
2. The installation is portable.
3. WARNING: The game installs in the default folder (Program Files). After installation, the game directory can be manually moved to any location. If you don't have program installation rights, the installer may report success and don't do a thing otherwise. Install the game at home and copy the game directory to a portable drive if you plan to play at work.
4. PROTIP: There's an Effects slider if you're annoyed by shininess or playing a survival level.
5. The scores are in plaintext.
6. Most of the plot is in plaintext too - don't go poking around text files if you don't want spoilers!

clicky

Their FAQ didn't say, any DRM? It looks intriguing.

My preferred TD game is Byteality tower defence. Plants vs. zombies and Immortal Defense and Defense grid were quite good, but Byteality is the one which I think I still like the most.

http://www.yoyogames.com/games/49892-byteality-tower-defense

Reason I like it over the others:
* The vector based graphics make for easy to discern enemies, and looks quite good even when there's a lot of effects on the screen.
* Different levels with multiple spawnpoints, exit points, paths and bases to defend. A fast enemy will not be as harmful to you as it passes by your base faster and has less time to do damage.
* Lots of different upgradeable towers, all in spite of being vector graphics looking better when upgraded (at least the attack does)
* Each level limits which towers you can buy and how many of each, so there's never a "generic solution". You have to make do with the towers you have and just make sure to place well and upgrade wisely.
* Special towers, not the standard "slowdown" towers, but instead towers whose sole purpose is to constantly generate currency for you, or towers which slowly heal your base, making it possible to leave the level with more health than starting.
* Timed levels: Each level has a time limit. The level isn't over until the time has run out, so you need to survive for that amount of time to win, but there's also a kill quota, so you might have to just keep pumping in new waves with the next wave button in order to make your quota on time. But put in too many, and the waves will be too strong for you before time is up.

Also, I'd like Dungeon Defenders to be done soon, but I'm starting to feel like the devteam is playing the Duke Nukem forever game. I'm not as hyped up over it anymore, it'll get out when it gets out. Until then, in the last 6 months there's been several action/tower defense games released. By the time they release it, it might be old news all around.

Also, honourable mention to Monday night combat. Combining a third person shooter with tower defense. Many see it as a rip off of Team Fortress 2, and some of the design choices can be annoying, but in the end I quite like it, blasting the opposition's bots before they get to you, and shooting down opposition to help your bots to get to the other base...too bad I suck at it. Not a game for everyone, but there's a bit of entertainment to be found there.

I just downloaded and played it, never tried the play online edition of it, so can't say if that's broken. But the download seems quite well working.

No, none at all, and the installation is portable. Not only that, but the creator specifically said he 1) doesn't mind people disassembling the game and looking at the code and 2) will release the game as freeware at some reasonable point in the future (I think five years since release).

Here's the EULA of v1.1 in its entirety:
"By playing the game you agree to the following: you can redistribute the demo but not the full game. All other rights reserved."

Fun fact: When I first bought the game ($14.95 then), I wanted to install it on another PC in what was then my "household" and didn't find a tl;dr EULA, so I just bought another copy. The payment provider (BMT Micro at the time) emailed me and asked whether I was serious:
"Hello. I found two orders for you for Immortal Defense. Is this correct? Did you need two licenses of this program?"

The creator doesn't have money for marketing, so it's one of those cases professional reviewers should be trusted, because the time they spent writing a (honest) review for ID is the time they did not spend writing a paid review or going out with their significant other.

Oh, and two more things I forgot:

1. WARNING: The game installs in the default folder (Program Files). After installation, the game directory can be manually transferred to any location and in fact is perfectly portable. I went "huh what was that?" but some people feel offended and violated when a program doesn't let them choose the installation folder.

2. The visuals, while pretty, can get distracting. The creator considers it a legitimate challenge, similar to reaction speed and strategic thinking ("bad visual conditions is a known factor in real warfare so why can't I put some multicolored awesomeness into my game" - not a direct quote), but if you're annoyed or playing a survival level, there's an Effects slider.

OMG I cannot stop play Immortal Defense. The story is sooooooo engrossing and the gameplay is sooooooo fun.

They got a release date on Dungeon Defenders yet? Definitely looking at getting that one on Steam.