Sanctum is more of a "pure" tower defense game than, say, Dungeon Defenders, in that there are lanes you can set up to manage enemy paths and kills, and less of a mass of enemies just heading straight for the "goal" like DD, and somewhat like OMD.
It has the speed of OMD with the added benefit of (up to) 4 player co-op, and with the recent updates you can choose weapon and tower loadouts to customize your experience. Unlike DD there is no persistence in your character, so game to game is new and unique, there's no "loot" or anything like that, there's just towers and guns.
*NOTE* Because the game has actually changed pretty dramatically with the update to new towers, weapons and loadouts, some of the following will be outdated, but the core gameplay is still the same.
What does Sanctum look like? Here's a couple shitty screenshots. Maybe if I think about it next time I play I'll take some of my own. Or you can take some and PM me and I can put them in the thread!
Yeah, while the game isn't going to win any graphical awards, those images don't really do it justice. It's an Unreal engine game so it runs well even on ancient machines and looks nice and pretty, with everything maxed out anyway.
If you want to see the game in action, here's a giantbomb quicklook and a totalbiscuit WTF:
Keep in mind the above note on the game updates above; that applies to both of those videos.