Chester is a city in the North West of England, UK. It sits on a sandstone ridge, originally amongst marshland. During the 1st Century, there was a Roman fort built atop the ridge, originally made from wood with turf ramparts, and later rebuilt in stone. At the South East corner, on the outside of the fortress, the remains of an amphitheatre was found during roadworks in the 1920's. Little remains of the structure, but the arena floor, and the outer wall foundations remain. From the footprint, it can be surmised that it had 3 phases of building. The first was likely as a ludus, a grassy training arena, then a larger stone and wood one was built, and finally, an even larger stone version that could house as many as 7,500 people was built.
This video shows how the Roman ampitheatre in Deva (Chester, Cheshire, UK) might have looked in the mid 3rd Century. It stood at the South East corner of the fortress, and could hold around 7,500 people.