Cross-section of an insulae (the Latin word for island). Concrete and brick building with wooden roofs. Built around small courtyards with shops and taverns on the ground floor and living quarters above.
Dome of the Pantheon [These are not purely decorative. The Romans, masters of concrete as they were, used progressively lighter compositions of concrete, starting with base gravel and eventually broken pumice at the top to scale the density of the material with the weight they supported. These are another weight shaving technique!]
The Romans constructed aqueducts to bring a constant flow of water from distant sources into cities and towns, supplying public baths, latrines, fountains and private households. Waste water was removed by the sewage systems and released into nearby bodies of water, keeping the towns clean and free from noxious waste. Some aqueducts also served water for mining, processing, manufacturing, and agriculture.
Roman engineering - the roads. The Romans built the finest roads in antiquity. A deep trench was dug, into which was laid a layer of flat stones; next, stones in mortar, topped by concrete or sand or more stones. Finally, cobbles were set in mortar. The edges, unpaved on both sides, functioned as sidewalks.
Examples include the Aqueducts of Rome, the Baths of Diocletian and the Baths of Caracalla, the basilicas and Colosseum. They were reproduced at smaller scale in most important towns and cities in the Empire.