This is a Roman gladiatorial arena probably inspired by magnificent ones such as Colosseum in Rome which back in the day (or to be more precise, AD 60-70) was used for gladiatorial battles. Basically it was the ancient version of entertainment television but obviously more brutal and something neither of today's people's stomachs would stand a chance watching if they were teleported back in a time machine.
The arena as seen in the images is beautifully made with a great deal of details. It's quite small though and the surrounding area is just made up out of a flat landscape.
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(Map name in-game: My Arena)