As from 1316, the Viscounts of Melun form an alliance with the Counts of Tancarville. New structural changes followed by several construction campaigns during the 14th century transform the old stronghold into the fortified castle we know today.
A fortified gate with a drawbridge reinforces the 13th century manor. A new dwelling is built in the courtyard. The back of its wall falls in to the initial ditch, replacing the old curtain wall. New building work (mid 14th century and 1371-1384) enables the extension of the surrounding walls and the reinforcement of the defences.
A new curtain wall and three new towers are built in the south. The old curtain wall, in the north, is raised and reinforced. The new keep, main tower of the castle, has a stair tower (door and portcullis still standing) and a latrine tower. The 6th level is the parapet walk, encircled by machicolations. The watch tower (with a parapet walk) and the archival tower (with a latrine turret) are a little less high. According to the desire of the Counts of Tancarville, Blandy becomes a place of defence as well as a residence at the threshold of the Hundred Years’ War.