Kotir was an aging fortress, made from the red sandstone from the quarry that lay near the River Moss in western Mossflower Woods. Prior to the events of Mossflower, the fortress had been abandoned and was discovered and inhabited by Verdauga Greeneyes, along with his family and army. After Verdauga's death, his daughter Tsarmina became the ruler of the fortress.
Kotir was built upon a large lake that served as the prison for the Gloomer. When the woodlanders realized this, they attempted to re-route a nearby river and flood the fortress. However, the water poured in at such a slow rate that technically it would not completely flood until many seasons later. It was described to be gradually falling to ruin even before it was flooded, but it was still fairly strong.
When the Corim learned that the flooding process would take entirely too long, they managed to use the Wuddship to dam the River Moss, and increase the rate of flooding. After the water had risen to the point where half of Kotir was flooded, the woodlanders used a ballista built by Timballisto to destroy the castle, leaving nothing but a lake. Later this lake became a pond, and Redwall Abbey was built over it.
In The Long Patrol, it was discovered that the south wall of Redwall Abbey was built over part of Kotir's towers. After the wall collapsed, several Redwallers explored the ancient fortress and discovered a riddle and riches left by Verdauga long ago, which were hidden behind a window. The treasure consisted of many weapons and other things made of silver and gold.
It is also noted that the cellar of Kotir was larger than Great Hall, which is an indication of the sheer mass of the castle.