I found this on wikipedia, and it is repeated in ManxNotebook:
The Douglas Town Commissioners also designed to alter the anachronistic architecture of Douglas, built during the era of fishing and trading, and no longer amenable or safe for tourists. The proportion of the Manx population living in Douglas was also expanding, with 35% living there by 1891. The Victorian and later modernisation of the town was achieved at the expense of the original maze-like layout of the oldest streets. These were cleared away in the new street schemes and slum clearances of the 1870s to 1920s. The town's infrastructure was radically altered in order to convenience tourists, and in 1878, the Loch Promenade was constructed. In 1870, there were 60,000 visitors annually, by 1884, this had grown to 182,000.
And comments from J. Brown, writing about Douglas in the 19th century (and also available on ManxNotebook):
Considerations of space enable me to devote myself to only a mere brief statement of other slumming parts of Douglas. Among these might be mentioned Water Lane, Society Lane, Fancy Street, Hanover Street, Bigwell Street, Little Ireland (or more generally known as " Little Hell "), Athol Court, Shaw's Brow, the " Back Street " (now known as Market Street), Quilliam's Court, Tynwald Court, the district still known as " Senna." In my young days these unsavoury districts, with their utter want of sanitary methods, were a distinct danger to the health of the town, and certainly contributed an undue proportion to the death rate. It is indeed something to be thankful for that the majority of them have been improved, off the face of the earth, or so altered and improved as to be no longer a menace to the public health.