Lincoln castle was a large and strong one that could hold its own against the most modern of siege weapons and house a considerable garrison. A force based in Lincoln castle would be comparatively safe from attack, yet could ride out to attack and raid enemy forces in the area. It was also a strategically placed transport centre. The River Witham ran inland from the Wash at Boston to Lincoln and although there were a few fording places there were no bridges. For individual travellers or merchants with pack horses the fords were good enough, but a sizeable army would have trouble crossing a large river by a ford. The wagons carrying the supplies, heavy armour and dismantled siege engines could not cope with bad road surfaces and muddy fords were quite impassable. Smaller forces could make do with whatever roads were available, but a royal army needed a bridge or a stone-built ford. Effectively the Witham between Lincoln and Boston was impassable to an army. Lincoln was the lowest crossing point on the river.
from "The Battle of Lincoln 1141" by Rupert Matthews.
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