Sanctum is by now means a bad game but it's not really a good game either. The multiplayer is a big plus for me being able to play with a friend. But Sanctum simply lacks content and at times feel likes one of the java browser tower defense games out there just in 3D.
It has the same ridiculous damage and creep life increasing. There's mostly only one "good" way to solve the maps and there are only 4 maps, 3 weapons and 4-5 towers coming with the game. There's more content if you're willing to pay for it. The developer decided to squeeze as much money out of the players as possible, charging you for even the tiniest bit of additional content.
In retro perspective I wouldn't buy Sanctum again. Especially not with much better games like "Orcs Must Die!" or "Dungeon Defenders" on the market.