Burgfried

Burgfried (Keep) puts one player in the role of defending a castle. The other one has to lay siege on it and capture the castle's keep. The game is mainly card based which mostly depict soldiers, siege machines and counter-measures for them. Soldiers can be used by both sides, while siege machines and counter-measures are obviously only useful for the attacker and defender respectively.
Gameplay is supported by a simple plan that shows the castle and the status (ok/destroyed) of its parts. The keep is the innermost part, so the attacker has to destroy several parts (e.g. moat, walls) before he can land the final blow. For each part a special action card and siege machine is needed. There is also a mechanism for starving the defender out.
After a game is won (attacker: starving out the defender or destroying the keep, defender: holding out for four weeks), roles switch.
The game was originally planned to be extensible, but I've never seen one. The main game is subtitled "Basis-Set: Ritter" (base set: knights).