The Scottish garrison on Bit Island was insignificant compared to the invading Vikings.
Godred and Sigurd had no intention of negotiating with the Scots in the city; they were prepared for bloodshed. Bit Island had no farms or villages, so if the Scottish garrison didn't surrender, they would surely come out to fight.
The adventurers from Scandinavia, the warriors from the Northern Islands, and the warriors from the Southern Islands needed to learn to fight together. This insignificant Scottish garrison was the perfect chance to practice their tactical coordination and whet their blades.
The plan was to wait for the prisoner army to break. Once the Scottish army began to hunt them down and broke formation, the main force would enter the fray. They would attack the Scots from the flank and rear, surrounding them and beginning the slaughter.
