Rochdale's built environment consists of a mixture of infrastructure, housing types and commercial buildings from a number of periods. Rochdale's housing stock is mixed, but has a significant amount of stone or red-brick terraced houses from the late 19th and early 20th centuries. Rochdale's Town Hall, seven large tower blocks (locally nicknamed 'The Seven Sisters') and a number of former cotton mills mark the town's skyline. The urban structure of Rochdale is regular when compared to most towns in England, its form restricted in places by its hilly upland terrain. Much of Rochdale's built environment is centred around a central business district in the town centre, which is the local centre of commerce.