Which emperor added the fourth tier of seating?

Emperor Domitian added the Colosseum’s fourth level of seating.

The Colosseum was begun under Vespasian around AD 70–72 and opened under his son Titus in AD 80. Construction and modifications continued afterward under Domitian, the third Flavian emperor. His work included completing the upper seating area and adding the building’s top gallery for less privileged spectators.

This uppermost seating level was known as the maenianum secundum in legneis, meaning the “second wooden section.” It was associated with ordinary poor citizens, enslaved people, and women, and may have provided steep wooden benches or standing room rather than the substantial stone seats found lower down.

Domitian also ordered the construction of the hypogeum, the underground network beneath the arena. It contained corridors, cages, lifts, and machinery for bringing animals and performers into view. Because the hypogeum replaced the earlier arena arrangement, it likely ended the Colosseum’s early use for mock naval spectacles.

Titus completed the original public building, but Domitian was responsible for important later additions, including this fourth seating tier.