Starting from Martian clay or other materials, a brick-maker can create a wide variety of structures and paved surfaces as well as furnaces and ovens for smelting, blacksmithing, glass-blowing, cooking, etc. The brick-making craft requires only other small-scale crafts for its equipment (blacksmithing for its iron tools), thus qualifies as a small-scale craft suitable for a frontier town (small and largely self-sufficient) economy.
Due to the 1/3 gravity, structures made out of brick can be much larger than on Earth, yet still hold up under their own weight and be easy to transport.
The brick can be made from regolith, plastics, fiberglass or composite materials. Regolith can be sintered at high temperatures. A mixture of molten plastics and regolith powder can be produced at moderate temperatures.