For those of you unfamiliar with the concept of tower defense, the objective is simple -- use stationary towers to defend your side of the board to make sure no monsters come in and reach your base. Towers can be usually made in different flavors, such as ice towers to slow down monsters, fire towers to damage monsters and burn them over time, etc. Towers can also be upgraded as the game progresses, as defeating monsters yields points you can spend on more towers or tower upgrades.
Runes of Magic takes this concept one step further by introducing player characters into the works. Player characters can also aid in the defense of the base, but they might be better used to build towers or upgrade existing ones. It's your call as you play the game, giving you many pathways to keep the monsters out while your enemy is placed under siege.
If you're itching to see Visdun Fortress in action, check out the teaser video over on the Runes of Magic website.