Crowne Plaza Hotel

In the 1990s, when excavating for the foundations of the Crowne Plaza Hotel, workers literally hit a wall. This wall, it turned out, belonged to the 10th-century St-Donaas church (where Charles the Good, Count of Flanders is believed to have been assassinated in 1127), which later became a cathedral. Construction was allowed to proceed, provided that the remains were accessible to the public at no cost. It’s occasionally closed for hotel conferences, but otherwise you’re free to go down at any time (within reason) to find old maps, paintings, tombs and respite from the crowds above ground.