The cellars can be found near the library on the ground floor of the Castle. The main cellars are used for storing wine. There's also a locked room that houses some unique equipment used by Ventin Thensbannald. A hidden door in the cellar leads to an area that's been flooded.
|Spoiler: The waters in the secondary flooded cellars can be lowered by using a device found beneath the fountain house in the Castle's courtyard.|