anyone heared of the Stronghold series? its a very fun strategy game that takes place in the middle-ages where you can build your own kingdom or castle. you can defend your empire by surrounding it with walls with archers on them, towers with ballista's and mangonels, and even with traps such as pits with spikes, and the list goes on. very fun and addictive for a while.
gameplay: http://www.youtube.com/watch?v=CJ-UIQm8deY