Grosnez Castle is one of those enigmatic places.
On the one hand, from the ruins we must deduce that a castle of some size was build on the Grosnez headland, and it must have been quite magnificent judging from the ornate carved stonework as well as fulfilling the requirement of medieval castles to be projections of power to any potential attacker and invader. On the other, we really don’t know exactly when it was built (in my view probably around the 1340s or 50s by the Seigneur of St Ouen, the King’s righthand man in Jersey at the time) nor definitively when it was pulled down (probably in 1373 or shortly afterwards when it was captured by the French).
On every old map of the Island known to exist, even those which are over 500 years old, Grosnez Castle is described as being in ruins. It was probably only ever intended to be a place of temporary refuge in case of attack and it could not have withstood a siege like Mont Orgueil in the east (it didn’t even survive an attack which lasted no more than a day!). There is plenty of archaeological and other evidence to support this and if you happen to read the information board there which suggests the contrary, don’t believe it as whoever wrote it didn’t do their research very well!
The one thing which is certain, however, is that its location is spectacular with views to all the other Channel Islands and France on a clear day. For that reason, when signal stations were build in the 19th century by General Don to communicate with troops across all the Islands, one was sited on the cliffs near the Castle.