Who gets it: Bosporans

Hail, Archon of Pantikapaion and Phanagoreia and Basileios of the Kimmerian Bosporos! A military innovation has occurred in your realm. Our horsemen have tested the oval Celtic shield, and they are pleased with the results. You may now recruit new cavalry units equipped with the Thureos from the royal barracks. May Herakles lead us to victory!

It fires when a settlement held by Bosporans is a huge city or larger.

The game checks this at the end of every round.

1 unit can be recruited once it fires:

Unit	Who can raise it
Thureophoroi Cavalry	Free Peoples