Castle Cornet has guarded the harbour of St Peter Port since the 13th century. It sits on its own small island connected to the main harbour by a causeway, and its towers and battlements are visible from almost everywhere in the town. The view back to the town from the castle walls — granite houses climbing the hill, the harbour filled with yachts and ferries — is among the best in the Channel Islands.
The castle houses four separate museums covering island military history from medieval times through the German Occupation of World War II. The daily noon gun is fired by scarlet-coated castle keepers in a ceremony that has been running since the 18th century. It is a small piece of theatre that the locals are proud of and visitors find charming.
Allow at least two hours. The castle is large and the views reward slow exploration. The maritime museum and the story of the Occupation are particularly good. Entry is reasonably priced and the castle is open most of the year.