Mercury exhibits long-wavelength topography which has probably survived for ~4 Ga. Assuming Airy compensation, the survival of the topography indicates that only certain combinations of crustal thickness and thermal structure are allowable. A dry diabase rheology allows a thicker crust than a dry plagioclase rheology. The existence of ancient faults places some constraints on the thermal structure. Unless the crust of Mercury is both as strong as dry diabase and is heated mainly from within, the crustal thickness must be ≤200 km. The faulting evidence implies that the concentration of radiogenic elements in the crust plus mantle of Mercury is at least 80% of the terrestrial value. Combined with previous studies of the long wavelength gravity and topography, the crustal thickness of Mercury is probably 100--200 km. Faults on Mercury are probably several times stronger than terrestrial faults.