Tower Defense?

DD, Defense Grid and OMD are good.
Sanctum... Hmm... Not sure, but i didn't play a lot.

I recommend 3 titles:
"Orcs Must Die!" is my favourite. It looks great and plays even greater. Those traps aren't the usual defense-turrets of other games and the setting is unusal for the genre aswell.

"Revenge of the Titans" is somewhat hardcore. Very difficult in the long run. It has some unique elements though (a tech-tree, resource-management between levels, randomized levels where you can freely build anything) and feels almost like an RTS.

"Monday Night Combat" mixes Tower Defense with a class-based FPS. If you are into games like Team Fortress 2 you should check that out.

Sol survivor is also pretty good is like defense grid

Assassin's Creed Revelations :P

Orcs Must Die, Dungeon Defenders, Defense Grid. I recommend you each one of these. Each one has it's charm and you will love at least one of them if not all.

DD is full of hackers, I cant recommend it. Otherwise its a good game.

Revenge of the Titans. I've been enjoying it since the second Humble Bundle came out.
Atom Zombie Smasher is also good. It's not really a tower defence, but some call it so.

I am addicted to Tower Defense games and try to collect them all :).

My favorie one is "Defense Grid". It gets very challenging and has a great look.
"Plants versus Zombies" of course... it's easy, but fun and has the best music ;).
"Pixel Junk Monsters" is the most difficult one I know, but one of the best. You feel like a hero when you make it through!

My favorite Flash games are the "Protector" titles. They might look ugly, but give it a try! But there are many others worth checking out, like GemCraft, Cursed Treasure and Desktop Tower Defense.

I was a great fan of tower defense... now i'm back with Revenge of Titans

i've played a good deal of revenge of the titans which is sort of fun but i'm also pretty bad at it.

i recently got dungeon defenders from a friend and have played the first 2 levels, it seems cool since you have a character you can control as well as towers you can build, but is a little hard to see what i'm doing. i've only played one class so far but there are 4 and they seem to have their own towers. there's also dlc that gives you 4 more classes.

i also played just the christmas level of sanctum, so not enough to have much of an opinion.

if you count plants vs zombies, that's pretty fun too! i just have one achievement left there...

Not very "unique", and not on Steam either, but Desktop Tower Defence is a classic and is one of my favorites. I especially like how you get to design your own map.

I must say Plants vs Zombies and Orcs Must Die. Both are AMAZING games.

Orcs must die is really fun, I just beat it the other day and enjoyed the whole experience quite a bit :)