Europa Universalis IV

Play a country which has fort level 2 in the capital (Most countries) without building a better fort. with some luck you will get a princess heir event rather then a regular heir. Accept her as an heir. and this achievement should unlock. Keep in mind that a princess as an heir will cause extra trouble when she in-heritage the throne, a civil war is highly likely due to low claim to the throne.

I played as Castile when I got it.