Under the Place Royale lie the remains of the massive Palais de Charles V, first constructed in the 11th century and upgraded over hundreds of years in line with the power and prestige of Brussels's successive rulers. Destroyed by a great fire in 1731, the palace was never rebuilt. Parts of the palace, and one or two of the streets that surrounded it, have been excavated; the underground site is a fascinating glimpse into Brussels's past. You can see the remains of palace rooms and walk up the steep cobbled rue Isabelle, once the busiest street in the city. Access is through the Musée BELvue, which focuses on Belgium history and its royal family.