Woah! I absolutely loved this one! Seriously, i'll keep it to play with friends!
First of all the sounds of the title screen made me laugh, and the 3D low-poly art is awesome (the only bad point about the graphics are the colours, i think they're kind of drab). Then this bluesy/jazzy music is very cool! It really sounds like we're in a party-game and we're going to have fun playing with friends!
As for the gameplay, i had to take a little time to get comfortable with the controls, but it's very fun once we've got it! Once again, i only have one negative point to bring out : i find it a bit hard to aim the enemy with snowballs. I often miss my target, but maybe i'm just bad at this.
We can elaborate different strategies, building a lot of forts or building less but upgrading them. This give the possibility to change our behaviour in order to take the opponent by surprise (for example by suddenly attacking the forts or by upgrading one), which make the game even more interesting to play with friends. I think each player will influence with his actions the way his opponent will react, and i like this relation the game put the players into. (In fact i only played with the IA so this is only how i imagine it, but i'll show the game to some friends as soon as i have the occasion)
I also praise the great amount of levels! It's a simple change but bring a lot of variation to the game! Finally your game really stick to the theme, since the forts are occupying space and we have to destroy little by little the arena in order to build them.