Castles, palaces and monuments — hidden gems

In Dublin a London plane tree has swallowed a cast-iron bench: the Hungry Tree in the grounds of King's Inns, a quiet botanical curiosity.

In the free park of Karlberg Palace, in Stockholm, lie the grave of Charles XII's dog, the Temple of Diana and a runestone.

Beneath Copenhagen's Søndermarken park, a former water reservoir hides a cavernous underground gallery of contemporary art, unknown to most visitors.

The mausoleum of the dervish Gül Baba in Budapest: a sixteenth-century Ottoman tomb on the Rózsadomb, free to enter and hushed among the roses.

In Karaköy, a minute from the ferries, a wholly underground mosque in the dungeons of a Byzantine fortress. Squat pillars and three tombs.

On the Golden Horn, in Hasköy, an 18th-century Ottoman kiosk-palace with Venetian mirrors and a museum of Turkish musical instruments.

In Berlin Mitte, the ruin of the Franciscan church from 1250: brick Gothic, free entry and art exhibitions, ignored by the crowds at Alexanderplatz.

In the City of London, a Victorian loggia with 54 tiles commemorates ordinary people who died to save others. A stone's throw from St Paul's.

At 16 rue Chaptal, Ary Scheffer's hôtel with its courtyard and garden. George Sand mementoes, a free collection and a tea room in Pigalle.