The L-plan tower originally had three vaulted rooms, linked by a corridor in the first floor, although one room now has had its vault removed.[3] There is a large scale-and-platt stair to the first floor, a turnpike stair in the south west jamb, and a private stair on the north.[3]
Careston Castle is notable for its chimney-pieces.[3] The one in the Hall has an enriched cornice, and an overmantel with the Royal Arms of Scotland.[3] There are fine chimney-pieces also in the dining-room, and the central and east bedrooms on the second floor.[3] These chimney-pieces are thought to be derived in form from designs in Jacques Androuet du Cerceau's Second livre (1561).[7] Campbell also argues that Careston incorporates two of du Cerceau's house designs in the formation of its elevation and plan.[8] Two wings of the building have been demolished.[3]
A report in late 2021 stated that the 15,700sq ft castle's principal accommodation consisted of "four fine reception rooms, a library, six main bedrooms and five bathrooms — laid out over the first and second floors". Much of the ground floor was being used for storage and the rooms on the third floor were unused.[9]