The entire city of Tokyo-3 was essentially built as a fortress to protect NERV from the Angel threat and to provide housing for all those who worked there. The city was designed with the Evangelions in mind as elevators, power sockets, and weaponry litter the city. According to Gendo Ikari in Episode 21 "all the resources mankind has to offer are being poured into that facility," it is truly the last hope mankind has against the Angel threat. There are 18 reinforced windows within the superstructure of Tokyo-3 that allow the GeoFront and NERV HQ to be bathed in natural light.