The Episcopal castle of Haapsalu is one of the oldest castles in Estonia. The entire compound on 3 hectares is like one big museum. Here you can find much of the kind that you cannot find elsewhere: the unique, 803-meter long circular wall; internal defence system with earth ramparts and trenches; the only Dome Church in Estonia preserved in its historical integrity and many other things that can be labelled as exceptional and unique. One can spend days in the castle studying the different layers of the wall from the 13th-16th centuries, discover details of stonework, stairways, master signs carved into stone etc. There are people who come to visit the castle and each time the castle offers them new surprises. The walls of the castle are like a book, which can be read, but in order to read you have to know the secret language and the key to this is the knowledge accumulated over the years.