went onto the pavement or statumen. Into or onto the nucleus went a course of polygonal or square paving stones, called the summa crusta. The crusta was crowned for drainage. An example is found in an early basalt road by the Temple of Saturn on the Clivus Capitolinus. It had travertine paving, polygonal basalt blocks, concrete bedding (substituted for the gravel), and a rain-water gutter. Obstacle crossings Romans preferred to engineer solutions to obstacles rather than circumvent them. Outcroppings of stone, ravines, or hilly or mountainous terrain called for cuttings and tunnels. An example of this is found on the Roman road from Căzănești near the Iron Gates. This road was half carved into the rock, about 5ft to 5ft 9in (1.5 to 1.75m), the rest of the road, above the Danube, was made from wooden structure, projecting out of the cliff. The road functioned as a towpath, making the Danube navigable. Tabula Traiana memorial plaque in Serbia is all that remains of the now-submerged road. Bridges and causeways Roman bridges, built by ancient Romans, were some of the first large and lasting bridges created. River crossings were achieved by bridges, or pontes. Single slabs went over rills. A bridge could be of wood, stone, or both. Wooden bridges were constructed on pilings sunk into the river, or on stone piers. Larger or more permanent bridges required arches. These larger bridges were built with stone and had the arch as its basic structure (see arch bridge). Most also used concrete, which the Romans were the first to use for bridges. Roman bridges were so well constructed that a number remain in use today. Causeways were built over marshy ground. The road was first marked out with pilings. Between them were sunk large quantities of stone so as to raise the causeway to more than above the marsh. In the provinces, the Romans often did not bother with a stone causeway, but used log roads (pontes longi). Military and citizen utilization The public road system of the Romans was thoroughly military in its aims and spirit. It was designed to unite and consolidate the conquests of the Roman people, whether within or without the limits of Italy proper. A legion on the march brought its own baggage train (impedimenta) and constructed its own camp (castra) every evening at the side of the road. Milestones and markers Milestones divided the via Appia