The five-storey Central Tower, or "main mast", was built in the 15th century and heightened in the 16th. It measures around 11 m × 9. 8 m (36 ft × 32 ft), and the walls are 2. 3 m (7. 5 ft) thick at the base. Each storey contains a single large room with a fireplace, a garderobe or privy, and numerous chambers within the walls. The storeys were originally linked by a narrow spiral stair, until a larger stair tower was constructed at the east corner in the 17th century. The castle's more important prisoners were held here. Men such as Cardinal Beaton, Archbishop of St Andrews and James V's ambassador to France, and Archibald Douglas, 6th Earl of Angus, regent of Scotland in the 1520s, would have had a reasonably high standard of living, including their own servants, while in prison. The basement is vaulted, and the roof is also built on a stone vault. The parapet was rebuilt in the 20th century, although the course of projecting corbels on which is stands is original. Outside the tower is a well.