The Odeon of Herodes Atticus is a stone theater structure and was built in 161 BC by Herodes Atticus in memory of his wife, Aspasia Annia Regilla. It was originally a steep-sloped amphitheater wide with a three-storey stone front wall and a wooden roof. It was used as a venue for music concerts and had a capacity of 5,000. At present time, the Athens Festival holds performances there during the summer months.
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Texts and images sources: https://www.wikipedia.org/, https://www.visitgreece.gr/