Bock Casemates are a vast network of underground tunnels and defensive galleries carved into the cliffs beneath Luxembourg City. They were first developed in the 17th century when Luxembourg was under Spanish Habsburg rule, and they were greatly expanded later by Austrian engineers. At their peak, the underground passages stretched for many kilometers and formed part of one of Europe’s strongest fortress systems. Much of the fortress was dismantled in the 19th century after the Treaty of London (1867).