Three days in Visby: too poor to rebuild, which is why the medieval town is still standing

Walk the wall first, on the outside. It runs three and a half kilometres, stands about eleven metres high, was built through the thirteenth and fourteenth centuries and is one of the most complete medieval town walls anywhere in northern Europe. Thirty six towers are still standing and three of the main gates survive. You cannot walk along the top, but the path round the outside takes an hour and gives you the shape of the town, and the stretch on the north side facing the water is the best of it.

Just outside the east wall is the field where, in July 1361, a Danish army under Valdemar Atterdag met the Gotland farmers who had come out to fight them. About eighteen hundred of the defenders were killed. The mass graves were excavated in the early twentieth century and the bodies were still in their mail, which is why museums across Europe now know exactly what a fourteenth century farmer wore to a battle he was going to lose.

Inside, the cathedral. St Mary's was consecrated in the 1220s as the church of the German merchants, has three distinctive towers, one square and two round, and is the only one of Visby's medieval churches still roofed and in use. Everything else fell into ruin after the Reformation.

Finish up on Klinten, the limestone shelf above the lower town, which was the workers' quarter. It is a tangle of small wooden cottages, steep lanes and gardens, roses growing up everything, and the view down over the roofs, the ruins and the sea is the picture of Visby.

Start with the island museum, because it makes sense of everything else. Fornsalen holds the picture stones, which are carved and painted slabs from the fifth to the eleventh centuries covered in ships, horses, warriors and scenes nobody has fully explained, and Gotland has more of them than the rest of the world put together. It also has an extraordinary quantity of silver: the island's soil has produced more Viking Age hoards than anywhere in Europe, mostly Arab dirhams that came up the rivers from Baghdad, and the display is arranged so you understand how far the trade reached.

Then walk the ruins. St Katarina, on the main square, is the one everybody photographs: a Franciscan church begun in 1250, abandoned at the Reformation, and now a shell of pointed arches open to the sky with the sun coming through where the roof was. The others are spread through the streets north and east of it, unfenced and free, and in most cases you simply walk in.

Finish in the botanical garden by the north wall, laid out in 1855 by a local society, which contains mulberries, walnuts and a ruined church of its own, and is entered through an arch known locally as the Door of Love. It is where the town sits in the evening.

Visborg stood on the high ground at the southern corner of the wall, built around 1411 by Erik of Pomerania, who was king of Denmark, Norway and Sweden at once and later became, more or less openly, a pirate operating out of this castle. It was the strongest fortress in the Baltic for two centuries. When Gotland passed to Sweden the Danish garrison blew it up in 1679 rather than hand it over intact, and what is left is a low set of foundations and fragments on a green mound with the harbour below. It takes twenty minutes and the view is the reason.

The art museum in the old town runs changing exhibitions in a medieval merchant's house, which is a good hour if the weather has turned, and the ground floor rooms are worth seeing in their own right.

The ferry terminal and the bus station are both just outside the wall, and the airport is ten minutes by taxi.