[pjc50]

Well, you can't start a fire in the atmosphere. So either you have one inside, or (more likely) you'd use induction heating to do the forging.

Concrete requires some rather specific calcium chemistry. But basalt is quite a good building material on its own, especially if you adopt Inca building techniques of mortarless flat-surface construction. Then all you'd need to do is construct interior insulating airtight shells to live in.

There's plenty of iron on the red planet, and from the red dust comes oxygen. Nitrogen will be harder to obtain.