Nothing special here, but it is a fun little bit of multiplayer silliness. Short play sessions are the key.
The game is playable single player, but really isn't very much fun that way. Multiplayer is quite entertaining though and the more the merrier. The game scales the difficulty for the number of players, so if everyone is of comparable skill level the challenge level will feel about the same as single player. If there are weaker players in the game the game will feel significantly harder (although a skilled player or two can make up a lot of lost ground for their less able comrades).
The game starts off very easy, but builds in difficulty to pretty insane levels, so as long as you keep a high level save file you can always find a way to challenge yourself.
If you are a veteran of tower defense type games there won't be anything new here for you at all (and the game may seem too simplistic compared to some of the other tower defense titles out there), but if you are new to tower defense games this is a good starting point. This is a worthy addition to your collection if you want a quick, light, bit of multiplayer fun, but stay away if you prefer to exclusively fill your gaming time with more serious fare.