Castiglione del Lago has evolved on what used to be an island - the fourth island of Lake Trasimeno, in its south west region. Over the centuries, as the town grew, the flat gap between the island and the shore was filled with piazze, houses, churches and other buildings.
The newest parts of the city are at some distance from the old, so the centro storico (historical center) of Castiglione del Lago is a well preserved medieval locality that seems to be governed by a "law of threes". In the town walls there are three gates, and inside the town there are three piazze and three churches.