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!]
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.
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.