Crusader Kings III

A quick way to get this achievement is to convert/reform your faith into one which has 'Megalithic Constructions' as a tenet. This has no upgrades, so as soon as you build this in your domain you should ping the achievement.

There is literally nothing on this achievement recorded anywhere I have scoured the internet and therefore am happy to inform you I have just unlocked it now so will share my findings.

To get this you must fully upgrade a duchy building. This is the special building slot that appears beneath your normal holding buildings.

There a few pre-requisites required here:
1. You must actually own a Duchy or the special slot will either not show or be unavailable to select.
2. You must unlock the relevant culture fascination before it is available. This can take a long time, but it’s made considerably quicker if you are the culture head and pick the culture focus yourself. Decide the building you want to focus on and look for the culture requirements when selecting the build option.
3. Money. You will need quite a lot of this to follow the path to the top and ideally you want an empire so that money is no issue.

I got this around 1280 with an empire of 300+ holdings.

Any questions please ask. Just wanted to get a guide out to help people