The monument consists of the sixteenth century remains of Seagate
Castle and its adjoining courtyard.
The house which faces the Seagate in Irvine is roughly rectangular on plan with three external towers (one triangular and two circular
stair towers) projecting to the E. It has external dimensions of approximately 30m NW-SE by 12m NE-SW and 10m high to the wallhead,
with gables about 12m high. The two storey building is of mortared rubble and has crow-stepped gables. Its ground floor is vaulted throughout. The ground floor vault in the S is covered over with
turf. Only the N portion of the upper storey is intact. At the centre
of the facade is a round-arched doorway with late medieval
decoration. From this entrance, a passage with guard-rooms on either side leads to the rear courtyard. Communication with the upper floor was possible by the S stair towers (the S-most probably ascended to
the hall). The surviving upper rooms open through each other and
contain fine fireplaces and windows with cable and dog-tooth
mouldings. In the N section of the ground floor is the kitchen. Waste from the kitchen and first floor guardrobes drained into a cesspool
in the courtyard. There has been a range of buildings along the N boundary wall.
The area to be scheduled is irregular and measures a maximum of 45m
E-W by 40m N-S to include the castle and rear courtyard, including
the boundary wall which defines the area on all sides except the SE,
as shown in red on the accompanying map.