This is a water-powered mill for making spades and shovels, still in working order and maintained by the National Trust.
© OpenStreetMap contributors
The castle was built in the 17th century, rebuilt in 1813, burnt down in 1922, and the ruins were cleared away in the 1970s. The buildings that remain illustrate its late Georgian concept of Tudor-Gothic: there's an Italianate stair tower, a Barbican gatehouse and a stable block - this has been renamed Clotworthy House and now hosts weddings, conferences and the like. The chief attraction is the restored 17th century Anglo-Dutch water gardens. The main entrance is from Randalstown Rd to the north, but from town centre it's easier to enter through the Barbican from High St. Limited parking here, use one of the other town car parks.
The first castle of 1345 was called Eden-duff-carrick; it was replaced in the 17th-century Plantation era. This second castle grew and grew, and was still incomplete when it burnt down in 1816. That's the shell that you see today. The grounds are a working farm plus event space - society wedding or steam traction rally, take your pick - and not otherwise open. But it was a location for Game of Thrones so it's on the circuit for GoT tours.