One of the largest and most important departments of most royal courts is the avenary - the stables and horses of the royal family. As such, even smaller keeps and castles often have a robust stable, and the person who oversees this operation is usually called the avenor, the head groom, or the stablemaster. This person manages the stables and mounts for the party and units in the keep or castle.
The stablemaster's shop in a stronghold is the Stables. Horses under the care of a good stablemaster can move at twice the travel pace (as outlined in the PHB on page 182) for a day per level of the Stables before needing to take an extended rest. Also, having a well kept stable can attract nearby horse owners who will want to stable their horses there or make use of your services. This business generates a modest income of 50gp per month per level of the Stables.
Especially exceptional Stables can also improve the Attack and Power of Cavalry units: