Answer:
Representative Democracy.
Bathhouses, important precursor to flush toilets because instead of defecating in a pot and throwing it out the window or something, you would go into a hole actually resembling a toilet and it would go down into tubes filled with water similar to a modern sewer.
Similarly, aqueducts. If there's one thing Rome was good at, it was moving water around.
Architecture, including arches to aid weight distribution and conserve materials because a giant arch takes fewer bricks than a giant wall, domes which were basically an arch bent into a circle and served a similar purpose but for cielings instead of walls, and roads that weren't just dirt but instead "paved" (basically just a stone path) making them flood resistant.