Located throughout the former Empire, enduring examples of Roman architecture serve to remind us of the wealth, power, and influence that Rome spread throughout its domain. Much of the Romans’ architectural mastery is due to their use of concrete. Mixing a dry aggregate with a mortar that would take up water and then harden gave the Romans a range of building materials of great flexibility and strength. Roman concrete is very similar to modern Portland cement. Emperors cemented their reputations with grand public works. Roman architecture is responsible for the first large bridges that have lasted for centuries. Romans were the first to use concrete to build bridges. The largest surviving triumphal arch is the Arch of Constantine, completed in 315 AD to celebrate the Battle of Milvian Bridge.