Bock Casemates

This rocky outcrop is where Luxembourg began. In 963, a count named Siegfried traded some land for a small old fort standing here on the Bock promontory, high above a bend in the Alzette river, and built himself a castle. The deed calls it the Lucilinburhuc — the little castle — and that word became Luxembourg. That castle is gone now, but the cliff underneath it is riddled with tunnels, and that's what you're looking at.

The tunnels — the Casemates — were carved into the rock starting in the 1640s, first by the Spanish, then massively expanded by the French and later the Austrians, who turned Luxembourg into one of the most heavily fortified cities in Europe. The galleries under this rock alone could barrack twelve hundred soldiers, with twenty-five gun positions along the main passage and a well cut forty-seven metres down through the stone. After the fortress held out against a French siege for seven months in 1795, a French engineer called it the best in the world apart from Gibraltar — and "the Gibraltar of the North" stuck.

Most of the fortress above ground was demolished after 1867, when a treaty declared Luxembourg neutral and ordered its walls torn down — sixteen years of work. The Casemates survived because blowing them up would have brought down the streets and houses built on top of them. You can still walk through part of the network today, and from the openings cut into the cliff face you get one of the best views in the city, straight down onto the Grund and the river below. It's the reason UNESCO listed Luxembourg's old town as a World Heritage Site.