The third advance, which came to use by the Romans, was the dome. The dome, like the arch and vault, was widely used in many grand structures. It was a spherical vault, which rested on a base wall. Compared to the vault, it was more stable; however, it was limited because it thrusts outward in a circle. The Roman hemispherical domes were very impressive and beautiful; The Pantheon's great dome, built by Emperor Hadrian, is more than forty-three meters in diameter. Along with a revolution in structure and design, the Romans were the first to use and produce concrete; as well as marble, mosaic, and stone. The mortar mix the Romans used provided a very strong foundation and support for structures. They used decorative stones as well as marble in public baths and many buildings. The Romans vast building supplies and their confidence in their abilities to use them were instrumental in their architectural excellence. The durability and beauty of the Roman's arches, vaults, and domes speak for themselves two millenniums after they were built. In many instances, the vaults and foundation arches are all that is left of ruins. Up until the nineteenth century, arches and vaults were the only methods, other than post-and-lintel, for spanning space between walls and piers, and for constructing ceilings. Their appeal and widespread use up to this day, is due largely to their durability and efficiency, as evidenced by the Roman architects. Every great empire confers with a marvelous city, and Rome was no exception. The center of all trade, art, commerce and court for the entire empire, Rome was a magnificent city. Set on the shores of the Tiber River, Rome had been inhabited for nearly three thousand years. It was not until during the 7th century B.C. that it came to be called Rome. It was set in a perfect strategic location for it was not too close to the sea and was surrounded by seven huge hills. Unlike many smaller Roman cities, Rome did not have a very orderly layout.