Way way back I got talked into getting Steam at the beginning of the winter sale season. 2010, I think, the year I gave up the WoW and returned to the fold of the glorious PC gaming master race, gpu under one arm, keyboard the other, mouse dangling from my twitching hands. Glorious discount gaming master port gaming race. Blargh. The first game I got was gifted - Shatter, which is a nice little game - but the first one I really got into was Defense Grid. I think I paid 2.50 for it originally, with all the (then) DLC packed in and I've given them money twice more for DLC. Hilariously enough, this is the first tower defense game I'd ever played and probably the first time I'd really gotten into the genre. I think I'd played maybe ten minutes of it on a mod or something. I was never big into RTS games, and then I played WoW and Magic for a couple years, so I'd missed a fair bit of the middle years of gaming.
So Defense Grid was my introduction to tower defense. Since then I've played two of those Orcy games, Sol Survivor, Plants vs Zombies, Sanctum, Dungeon Defenders and perhaps some other shit I've forgotten about. Of all of those ... Defense Grid is the only truly pure TD game. Sol Survivor is close, as the interaction is limited, but the other ones all dumb down the TD mechanics in exchange for more action elements, for better (Orcs Must Die) or worse (Dungeon Defenders, ugh). Ironically, or not, there's a reverse bell curve in action; the more action game, the worse. Then the better!
Defense Grid is very, very pure TD. The tension doesn't come from running around or fighting Orcs, but in very carefully aligning the load balancing of the grid. Doing a fight well requires increasing amounts of precision, on two planes. First, when and where aliens die impacts your "recovery" rate, which is both in terms of interest, access to interest and a couple other little things. Second, the way your towers are structured - the build of the maze - relies on committing to the plan and guiding it as it goes. A couple towers spin into a maze, and then the maze's parts align together to do force multiplication until you have killzones that dispatch ten times the number of aliens the towers would if they weren't working in tandem. I'll talk about this again in a moment.

It's not a visually stunning game, though its art direction and conveyance is pretty solid. The locales are extremely varied, lending a ton more flavor to the game than it would possess otherwise. Maps include icy canyons, various high tech installations and the edges of future cities. The audio direction is really good. You're paired with an AI that ... Well, honestly isn't much more than a cheerleader, but he talks and rambles, gives some back story to aliens being pew pew'd by towers.