Boon of Incredible Foremanship (5e Epic Boon)
From D&D Wiki
Boon of Incredible Foremanship
Each day of downtime that you spend overseeing the construction of a stronghold decreases the construction time by 10 days. In addition, you can initiate construction of a stronghold in a location that would not normally support that type of building, for example: in a swamp, the side of a cliff, the top of a mountain, among the branches of a redwood forest, or on the surface of a lake. If you do this, construction proceeds at its normal rate.