This is a map based on the Borogravian assualt at the end of Terry Pratchetts book 'Monstrous Regiment'. The basic story is that the Borogravians need to recapture their main fort, which has been captured by Zlobenia and Ankh-Morpork. Some Borogravian troops have managed to get inside the keep, and have released the prisoners kept there. They are now using this small army inside the castle to help the main army attack.
This is basically what happens. There is a bit more detail in the game, and even more detail in the word file that is in the folder.
Special thinks to Terry Pratchett for writing the book 'Monstrou Regiment', which this map is based on.