• 214-step stone staircase links the Lower and Upper Fortress, a long, curving climb to plan around with a stroller
• Reversed clock hands on the Clock Tower: the large hand shows hours and the small one minutes, so boats on the Danube could read the time
• EXIT Festival venue, since the fortress hosts one of Europe's major music festivals, so check the festival dates before you go

Petrovaradin Fortress was built above the Danube between 1692 and 1780, on three levels. Kids can climb the 214 stone steps to the Upper Fortress, find the Clock Tower whose big hand shows the hour, and look across the river to Novi Sad. Underneath run more than 16 km of military tunnels. The Arsenal now holds the City Museum of Novi Sad, and a bastion holds the Planetarium.