the reason i think that there arnt that many comercial games are because of blizzards battle.net World Editor were you can make tower defence with graphics as good as horde of orcs in fact i think i recall playin a game just like it on battle.net

and incase i confused you its Warcraft III Frozen Throne's battle.net

so if you like tower defence games thats the place to go there are crap loads of them and being someone that played prety much all of them to date the best one ever(my opinion) is Gem TD.

Note: until this post i thought TD were invented by Blizzard learn somethin new every day on this forum