An Caisteal (English: The Castle) is a hillfort located on the Inner Hebridean island of Coll. The fort is located at grid reference NM17185815. Nearby is Dùn Foulag, which was once stated to be a dun, but is now considered to be only a naturally rocky knoll.
The fort is known locally as An Caisteal, which translates into English as "the castle". It is located at grid reference NM17185815; situated about 300 metres (984 ft 3 in) west-northwest of Ballyhaugh farmhouse. The site is within view of Dùn Beic to the south; and Dùn Foulag, which is 1,200 yards (1.1 km) to the north-east. Dùn Foulag is located at grid reference NM17505908. The early 20th century antiquary Erskine Beveridge considered Dùn Foulag to be a dun and thought he found evidence of a defensive wall. The current opinion, however, is that it is just a naturally-rocky knoll. This once supposed dun was visited in 1972 by the Ordinance Survey (OS) who could find no trace of antiquity on site; and currently the Royal Commission on the Ancient and Historical Monuments of Scotland (RCAHMS) classifies it a 'natural feature'. What Beveridge thought was a defensive wall is now considered to be the remains of a modern bulding; what he thought were "fallen stones" are only beach stones.
Traces of the fort are visible on a rocky boss, which reaches 12 metres (39 ft 4 in) above the surrounding ground. Three sides of this cliff are extremely sheer; in some places all the way, 12 metres (39 ft 4 in), to the ground below. The rock is inaccessible on these three sides; although it is accessible from the north over a natural terrace. This terrace is a narrow shelf which ascends around the cliff-face to on the north-west of the cliff.
The fort's main wall occupied the crest of of a terrace, on the north and east sides; on the south side, the wall followed the cliff-edge. The main wall encloses an area of 29 metres (95 ft 2 in) from the north-west, and transversely 26 metres (85 ft 4 in) to the south-east. According to the RCAHMS, only a small proportion of this area would have been inhabitable, since large areas of this space are covered with bare rock. Only a band of rubble marks the wall in the east, although the west and south-west wall is in places 0.5 metres (1 ft 8 in) high. There are traces of an outer wall which would have guarded the easiest approach to the fort. This wall lines a part of the lower terrace on the north-west.
When Beveridge visited and documented An Caisteal he found one of two "doubtful" hammerstones at the site.
