Kaiseraugst -- it's THREE !

[Simplex]

.....Amphitheatres, that is. 
The youngest one -- 4th century.
Nearer to Castra Rauracense.
50 x 40 m outer boundaries.
Built inside a quarry near river rhine.
One outer stone wall, the rest is wooden.
See pics.

https://www.watson.ch/schweiz/aargau/965...t-entdeckt

https://www.ag.ch/de/aktuelles/medienpor...180549.jsp       (Far better pics !)

Greez