The fort was 'motte and bailey' in design meaning that it sat on a hill (the motte) and at the foot of this a ditch had been dug (the bailey). The combination of incline and ditch effectively raised the structure even further from any invading hordes. The Huntingdon remains show there was a causeway of sorts running into the keep, which would have been bounded by a wall of wooden stakes.
Huntingdon was founded by the Anglo-Saxons and Danes. The area regularly flooded until the 17th century when it was drained by Dutch engineers. The fort, therefore, would have stood above the surrounding fenland and formed an island during the wet season. You can just imagine visitors plodding through the marshes to be greeted with an imposing structure rising majestically out of the damp mist. They would then climb up the causeway and into the settlement, safe from any marauders on the flatlands.
Map image from this site.