The medieval castle at St Suzanne
If you thought that William the Conqueror confined his warmongering to mainland Britain, you were wrong. The castle at St Suzanne is the only one that he tried to take and failed, in a seige lasting four years starting 1083. By this time however he was viewed as English, and the camp where he based his attack, just under a kilometre away, is still called the English camp (Camp des Anglais). The castle did eventually fall to the English, but not until 1425, and then only for a period of 14 years.
The castle site is open to the public for a small charge, and is the focus of a visit to this medieval town which should also include the Musée de l'Auditoire (open April-September from 14.00), as well as a browse of the excellent gift shop, and the bar for a coffee and/or snack. There is also a tourist information office, worth a stop to find out what's on.
While you are in the area it is worth making the short detour to see the camp des Anglais, known also as the camp de Beugy, a little way to the North of the town on the D143. This is essentially a fortified field, and is the camp from which William the Conqueror launched his attack. Also worth doing is the walk signposted to the Tertre Ganne, which is the cliff opposite and which presents a spectacular view down on the castle.
There is a charge for admission to both the castle site and the Musée de l'Auditoire. St Suzanne is almost directly North of les Hallais, about 20 minutes away by car. Allow half a good half-day for a leisurely explore.