You'd have to devise a really good plan to get access to a roof on a high-rise building. Hypothetically, someone could hide out on the floor that has the roof access door, wait for a guard doing his rounds, if they still do that since everything is camera now, knock out the guard and take his keys to unlock the access door, and get up there that way. Sounds complicated to me. Detroit has several tall bridges. You could always climb to the top of one of them if you can get by the suicide blockades that they put on to keep people from being able to do that.