From the back of the box:
1289. King Philip the Fair wants to have a new castle built in Caylus. Workers and craftsmen are flocking by the cartload, attracted by the great prospects of such a formidable task. All along the road that snakes from the foot of the castle, a city is slowly rising up.
The players embody master builders. By building the King's castle and developing the city at it's foot, they earn prestige points. When the catle is finished, the player who has earned the most prestige points wins the game.