Going to Mars is easy. We've even done it twice in the last 5 years.
Making a self-sustaining city on Mars is impossible. And even if not impossible, certainly costs many tens of trillions of dollars, which is as good as.
Impossible is pretty strong. Who knows, maybe there's material that can be used for construction buried 100 feet under the surface. Or we figure out how to build stuff with Mars dirt. Humans are the most resourceful and adaptable beings in the universe (that we know of) and life, uh, always finds a way.