Haapsalu Castle — or the Episcopal Castle — is a vast thirteenth-century fortress built as the seat of the Bishopric of Ösel-Wiek. The main keep, tower, and surrounding curtain walls are remarkably well preserved, and a cathedral with a nave dating to around 1270 stands within the enclosure. It is one of the best-maintained medieval fortifications in Estonia.
The castle museum inside covers the history of the Ösel-Wiek bishopric and the development of Haapsalu as a regional centre. The open-air courtyard is used for concerts and theatrical performances in summer, most notably the White Lady Days festival in August, when the full moon illuminates the chapel window and the legend of the White Lady is re-enacted. The tower offers views over the old town and the bay.