Well, the parts I can remember right now in a short and not complete version. for a time the fortress was held by a coalition force from the various cities of the Moonsea to protect the borders. Zenthil Keep, wanting to take control of the castle, manipulated the ogres of the area into attacking and tricked the main force of the keep consisting of soldiers from the other city's, out on the field. They were massacred when they were pulling back and found the gates of the castle blocked. The remaining non- zenthish forces in the Citadel were then massacred by the zenthilars. I think this happened in the late 1330's.
As I said, this is a short incompleat version. I would suggesst getting the prior mentioned sourcebooks and the 2ed Moonsea book.