Chateau des Marais, Medieval castle in Saint Sampson, Guernsey.

Chateau des Marais, known locally as Ivy Castle, is a medieval castle on a small rocky hillock about a hundred metres from the Channel shore, between St Peter Port and St Sampson. An irregular motte with a flattened top is enclosed by a moat, an inner and outer bailey, and a low external stone wall. The stone fort visible today dates from the Napoleonic-era refortification.

Neolithic settlers used the mound between 3500 and 1500 BC, and a fortified structure stood here by 1170, already called old in 1244 and 1292. In the 13th century the mound was flattened into a motte-and-bailey with a moat, and the marsh was drained by a channel cut to Belle Grave Bay. After Castle Cornet was built in the mid-13th century the fort fell into disuse, and it was refortified in the Napoleonic era.

The name Ivy Castle comes from the ivy that covered the ruins when botanist Thomas Knowlton visited in 1726; he described walls covered with ivy, which he supposed gave the site its name. The castle is open to the public and reached by footpaths from Ivy Castle Lane or Victoria Avenue.

The castle is accessible via footpaths from Ivy Castle Lane or Victoria Avenue, with parking available near Sir John Leale Avenue. Wear sturdy shoes since the hilltop terrain is uneven and the moat area can be damp underfoot.

The 13th-century builders cleared the undergrowth by burning it, marked out the site with standing stones, cut turf and raised an embankment topped with clay and granite. Excavations in the 1970s found early jewellery, pottery and flints from at least the late Neolithic period.