The most important single discovery in recent years has been the uncovering of the Roman amphitheatre, in a place where no-one expected it. It was found under the Guildhall yard, in front of the medieval Guildhall. Only one end of the amphitheatre was excavated,  where the cellars for a new library were being dug out.

In the late Saxon period, the yard was used for a huddle of small houses, but it seems likely that when the Guildhall was erected in the Middle Ages, the remains of the oval depression were recognised. The Guildhall itself was built over one side, where the Roman President of the games would have watched the proceedings.
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The Roman amphitheatre. At the bottom, the two curving walls formed the walls of the arena. At the centre was the gate, and the twin walls  running to the top of the picture formed the entrance way.

The steel girders were needed to  enable to deep excavation. 

Amphitheatre drain

Still preserved under the entrance way to the amphitheatre, was the original timber drain, its wood perfectly preserved, and still carrying water.

There was even a 'silt-trap' whether the silt was encouraged to settle, and could be cleaned out.