The first castle built on the site was probably of an octagonal design, running along the top of the ridge, protected by square and semi-circular mural towers, with a barbican guarding the eastern entrance. It probably resembled White Castle, remodelled by de Burgh in the same period, or Bolingbroke and Beeston Castles, both built in the 1220s. The castle was built of Kentish ragstone and cemented by a mortar containing a large proportion of seashells, particularly cockleshells from the cockle beds of neighbouring Canvey Island. A valuable tidal watermill had been built by 1249, essential for making bread for the castle; recent archaeological work suggests that it was positioned on a wharf reaching out into the estuary just to the south-east of the castle. Terraced gardens were built along the south side of the castle overlooking the estuary, probably including vineyards.