Social elements such as wealth and high population densities in cities forced the ancient Romans to discover new (architectural) solutions of their own. The use of vaults and arches, together with a sound knowledge of building materials, enabled them to achieve unprecedented successes in the construction of imposing structures for public use. Examples include the aqueducts of Rome, the Baths of Diocletian and the Baths of Caracalla, the basilicas and perhaps most famously of all, the Colosseum. They were reproduced at smaller scale in most important towns and cities in the Empire. Some surviving structures are almost complete, such as the town walls of Lugo in Hispania Tarraconensis, or northern Spain.