Kojirase Outaishi to Yakusoku no Himegimi

Leylia is a tomboyish princess from the small country of Barcos. In order to protect her country from a vicious superpower, she accepts an offer of marriage with a neighboring country. However, the crown prince has a lover, and he has given her ten unreasonable conditions, including: only a fiancé for the time being, not to appear in public, not to talk to each other... etc. Leylia intends to fix the attitude of the haughty crown prince who takes advantage of his superior position... But then, for some reason, she and the crown prince suddenly got close...? The princess of a weak country makes a selfish crown prince cry foul! A royal palace love fantasy!

(Source: Kadokawa, translated)