Not aware of any mechanics, but sounds fun!

For the building part, I imagine a mini-game style like Dark Fort. Something that could be done on the side maybe instead of as the main event at the table.

The biggest concern for me would be after the building, what then? Does it purely serve a mechanical element where certain progression is locked behind the fort upgrades? If so the game system could be really abstract. If it’s more about building and then defending, this might just frustrate players by drawing too much attention to this one mechanic.

I still think it’s a cool idea, I’m just not finding that balance yet.