'Enormous force' collapsed Iron Age broch in Assynt
Archaeologists are trying to understand how and when a 2,000-year-old building called a broch in the north west Highlands was left in ruins.
The outer wall of the Iron Age tower was constructed using stones weighing up to 100kg each.
The broch, today a huge pile of rubble, may have been up to 13m (42ft) high.
John Barber, lead archaeologist at the dig at Clachtoll in Assynt, said "enormous force" would have been needed to shift the stonework.
He said: "Engineering calculations will give us more accurate measurements of the tower, and we found radiocarbon datable material among the rubble which could give us a date for the collapse.
"We also see that the building has been remodelled and reworked, but without further excavation we cannot know when or by whom."
Bones, pottery shards and charcoal have been dug up during the excavation, and will be taken for analysis.
Group Historic Assynt believe Clachtoll broch was built and occupied by a sophisticated maritime culture stretching up to the Northern Isles and out to the Hebrides at a time before the Roman conquest of southern Britain.
The tower may have been seen by ancient Greek geographer Pytheas during his circumnavigation of Britain.
Remains of brochs have also been excavated in Caithness and on the Northern Isles.
Historic Assynt is planning further excavations at other sites - a Neolithic chambered cairn at Loch Borralan and a pre-Clearance longhouse in Glenleraig.