MadalinStroe: In fact I don't think there can even be such a thing as a
multiplayer tower defence game. There can only be a
co-op tower defence. Unless the OP meant DOTA/MOBA but incoreclty called it tower defense.
adaliabooks: In that case yes, a MOBA with tower building is basically the only way I can see of doing competitive multiplayer TD.
Competitive tower defense already exists, and is not really related to MOBA ;-)
Each player plays in its own area, build towers and earn gold by killing creeps as usual. But there are two ways to spend gold: buying towers (of course) or buying creeps to add them to the next wave spawning in your adversaries areas.
The winner is of course the last player standing.
Some games to spice things up will make the next wave spawn as soon as one player killed all of the creeps of the current wave, even if the other players are already struggling with the current one.