Short answers, pulled from the story.
Engineers erected semicircular arches throughout the Roman Empire, stretching from Syria to Scotland. The first use of these arches appeared in civic structures like drains and city gates.
Roman concrete known as opus caementicium led to the liberation of shape from traditional stone constraints. Builders could now create monumental architecture without relying solely on cut stone blocks.
The Mausoleum in Campus Martius became one of the major monuments built by him. It was made almost entirely of concrete using updated construction techniques.
The revolution reached its apogee in the architecture of Hadrian who instigated a burst of inventiveness. Buildings included Hadrian's Villa and the Pantheon in Rome.
The Pantheon held the largest dome in the world for more than a millennium. It remains the largest unreinforced solid concrete dome to this day.