The Pozzo di San Patrizio (St Patrick's Well) is one of the engineering marvels of the Renaissance: a deep well sunk 53 metres into the volcanic rock, with two interlocking spiral staircases of 248 steps each that allow you to descend on one staircase and ascend on the other without ever meeting the other traffic. Light comes in through 72 windows cut into the central shaft. At the bottom, a footbridge crosses the water.
The double-helix design was entirely practical — it was built to allow a continuous flow of pack animals carrying water up from the bottom without blocking the animals going down. It is also beautiful, and the view looking up from the bottom — the spiral of lit windows receding into light above — is memorable. Entry includes a spiral descent and ascent.