Heidelberg

Twenty-three stops on two levels: below, the old town with the castle, the Church of the Holy Spirit and the Old Bridge; above, on the Königstuhl at 568 metres, the state observatory and two Max Planck institutes. The university began teaching in 1386 and is the oldest in Germany never to have interrupted it.

The castle is a ruin by decision. French troops burned it down in 1689 and blew up what was left in 1693. Between 1883 and 1890 a commission of experts concluded that no reconstruction should be attempted and that only the existing fabric should be secured — the dispute is regarded as a founding case of German heritage conservation.

More than a million people make the climb each year. In the cellar stands the Great Tun of 1751, holding 221,726 litres; beside it lie the remains of the Hortus Palatinus, begun in 1614 by Salomon de Caus and never finished. The Old Bridge of 1786 to 1788 was blown up by German troops on 29 March 1945 and was open again in 1947. The observatory, founded in 1898, found more than 800 minor planets up to the 1950s. Since 1997 the Documentation Centre of German Sinti and Roma has recorded the genocide of that minority.