The German tactics used for urban warfare in Berlin were dictated by three considerations: the experience that they had gained during five years of war, the physical characteristics of the city, and the methods used by the Soviets. Most of the central districts of Berlin consist of city blocks with straight wide roads and contain several waterways, parks and large railway marshalling yards. It is a predominantly flat area, with some low hills such as Kreuzberg, which is above sea level. Much of the housing stock consisted of apartments blocks built in the second half of the 19th century. Most of those, thanks to housing regulations and few elevators, were five stories high and built around a courtyard that could be reached from the street through a corridor large enough to accommodate a horse and cart or a small delivery truck. In many places, these apartment blocks were built around several courtyards, one behind the other, each reached through the outer courtyards by a ground level corridor similar to that between the first courtyard and the road. The larger, more expensive flats faced the street while the smaller, more modest dwellings were grouped around the inner courtyards.