Kaer Morhen (Elder Speech: Caer a'Muirehen, meaning Keep of the Elder Sea) is an old keep where witchers of the School of the Wolf used to be trained. The name is a corruption of Caer a'Muirehen, revealing that there used to be sea around it, which is further indicated by the presence of fossilized sea creatures embedded in the stones on which it was built. However, by the 1200s it is located in the mountains of the kingdom of Kaedwen, right off the Gwenllech river.

The castle can only be reached by "The Witchers Trail", which is easy to miss, and encircles the keep. It is often given the nickname "The Killer" by young witchers.

In older times, witchers trained and underwent mutations here, but fanatics who considered witchers an abomination attacked the keep and killed most of the teachers and students. Vesemir was the last known witcher to live in the keep, although other witchers sometimes rested here between their assignments, often during the winter, after which they set out on The Path again.

In 1232, Eskel's surprise child, Deidre Ademeyn, took refuge here as those pursuing her didn't dare enter with the witchers around.

In 1270, Kaer Morhen was a ruin of its former self, with only a few witchers remaining. During this time, Triss Merigold was staying here with the remaining witchers when an unconscious Geralt was found near the fortress. Soon after he regained consciousness, the fortress came under attack by a criminal organization, the Salamandra, and the school's closely guarded herbs that were once used for the Trial of the Grasses were stolen and Leo was killed. After Leo was buried just outside Kaer Morhen, the witchers aided by Triss left in different directions to learn more about their attackers and recover the ingredients.

A couple years later in 1272, the witchers of the school once more met up here on learning that Ciri was back but being chased by the Wild Hunt. Yennefer arrived before Geralt to begin preparations, as she wished to use the first part of the Trial of the Grasses on Uma, an odd creature that was their only link to learning what happened to Ciri. After a successful transformation, Geralt headed out and soon brought Ciri back, but with the Wild Hunt right on their tail. During the Battle of Kaer Morhen, the group tried various tactics to defend their home, but were eventually overpowered, resulting in Vesemir's death, until Ciri's Elder Blood activated, threatening to destroy not just the Wild Hunt members but the entire fortress along with it. However, a weakened Avallac'h was able to stop her.

The remaining allies then held a pyre funeral for Vesemir before the witchers decided to go their separate ways, leaving Kaer Morhen behind.