In Visdun Fortress you have to fight against spawning monsters that try to destroy your home base.
Click the battlefield button and click Visdun Fortress to register for it. The game will notify you as soon as there is another player.
To get to the home base the monsters have to go through a small maze which is surrounded by foundations. To stop them you have to build towers on these. But beware - the monsters will also attack your towers, so don't forget the healing towers. With time you will also be able to upgrade your towers to keep up with the ongoing waves of monsters.
In total every team has 100 lives and the game will be over when one team has none left.