The casements are a honeycomb of tunnels and galleries first carved out in 1644 by the Spaniards and extended in the 18th Century by the Austrians. The total length of the tunnels, which are on different levels, was 23 kilometres, and the Bock Casements could hold a garrison of several hundred men and 50 cannon. Some tunnels were sealed in the 19th Century, but 17 kilometres remain. The fortifications and the Old Town of Luxembourg have been designated a World Heritage Site. Since 1933, sections of the casements have been open to the public.