Rome is a moderately seismic area suffering from amplification of seismic waves from earthquakes in the more seismic areas in the mountains. Basically all of It

Rome is a moderately seismic area suffering from amplification of seismic waves from earthquakes in the more seismic areas in the mountains. Basically all of Italy is seismic. Part of the Colosseum was destroyed by an earthquake in 1349. The google translation of this Italian page is quite good at explaining what happened https://www.focus.it/cultura/storia/perche-il-colosseo-e-crollato-per-meta https://www.focus.it/cultura/storia/perche-il-colosseo-e-cro... TLDR, the asymmetrical 4-2 storeys shape is the result of that earthquake when the top of the part of the building built on softer ground felt down.