The siege lasted four-months, a length of time unprecedented in the history of Viking warfare in the British Isles, and ended when the defenders ran out of water.

[4] Dumbarton Rock was an extremely strong defensive position and had previously survived several sieges,[5][6] including a co-ordinated assault by Angles and Picts in 756.

[7] Following his successful invasion of England, including the conquest of York in 866, Ímar joined forces with Amlaíb, the Scandinavian King of Dublin, to target this prosperous town.

No other campaign by Ímar and Amlaíb involved such a concentrated assault in one place, providing evidence of Alt Clut's continuing power at the time.

Based on these texts, historian Alan Macquarrie suggests that after a prolonged siege the Norse attackers may have seized the lower part of the rock, where the well was located, forcing the defenders to retreat to the highest craig without a water supply.

[2] Following the siege of Alt Clut's royal and religious centre, the kingdom moved 12 miles (19 km) upriver to Govan,[4] while Dumbarton Rock may have become a Viking outpost for a period.

Nonetheless, Strathclyde is documented in battles and royal records to have existed as an independent kingdom up until some point in the 11th century, when it was peacefully incorporated.