During World War III, the enemy dropped 2 nuclear bombs over Milan’s airports: Linate and Malpensa. Much of Milan’s population is thought to have survived these first hours of the war. However, many survivors were soon wiped out by total anarchy and the effects of nuclear winter. By 2033, almost 2 decades after the worldwide conflict ravaged the planet, the surface of the Earth remains uninhabitable. The snow has not yet melted, and the city’s ruins are trapped in an icy and silent wasteland. At first Milan’s metro system has provided some protection, but now only a few thousands live on and most of the network is abandoned.
Before the war, Milan's Metro was the biggest underground network in Italy; it counted 101 stations and 35 more under construction. Though the metro was not designed as a fallout shelter, many people managed to survive there for some time. At last, it was cold and famine which decided many survivors' fate. It is known that the "Alberti" faction once controlled all the interchange stations and maybe the entire network, but they were gradually pushed back at the southern stations of lines 1, 2 and 3. Now a few thousand survivors are still alive, and they constantly fight to defend themselves or to take over the whole metro. At some point, sentient humanoid mutants occupied Duomo station and the buildings above, including Milan's Cathedral. It is unknown whether they killed all the station inhabitants or simply let them escape, but no survivors tried to reclaim the station or even to pass through it since then. The station is known as "The Nest". Some stations are flooded or destroyed, but it is unknown when or why it happened.
Milan is a city filled with rivers and canals, both in the open air or entombed: water had always been a problem to deal with, but since the nuclear winter is still ongoing it is possible that many canals are frozen or dry. There is still water flowing underground however, since some stations of the metro are flooded and the survivors have no shortages of water, although it is contaminated. In The Children's Crusade there are some torrents and canals the older survivors know of: they remember some of them were polluted from the industries and houses (the Seveso torrent, for example), and some think their water is quite clean now.
By 2033 the nuclear winter has not faltered yet, and the city is trapped in ice and snow storms. The freezing climate keeps Milan and the bodies of its former citizens from decaying, so it's likely that the temperature are still very cold. Even so, if the novel relates directly to the Dmitry Glukhovsky's novels, the nuclear winter should be reaching its end in a few months or years. This is going to bring back the issues the city had with water for many centuries.