It is a team based tower defense, not solo. If 3 of your team mates let in 20 units and are destroyed, the game ends.

You can only win or even play by having enough players that don't suck too badly. It is fairly easy to get past the first few rounds. Quickly garrison all of your relics, have the trade cogs moving and the villagers mining. Build towers and recruit 5 or 10 troops.