Hidden buildings?

Is it possible to add hidden buildings in a settlement? If so, how?

And also how can I make a trait a requirement for the recruitment of a certain unit? (e.g. unit A is only trainable if governor has trait X)

Thanks

Hidden buildings aren't possible, you'll always see them in the campaign map screen.
And the second idea could only be done with a scripting workaround, it's not per se possible to use governor traits as a prerequisite in edb.

The only way you could have a (semi)hidden building is to have a prebuilt one that cannot be built during play.

It would need to not appear on the building browser either by creating an impossible conditional or by setting at the village level and making indestructible.

The town tab image would still exist but I assume this could be made invisible (though the space will still appear) though you could fill this to make it look like part of the interface.

The latter would be preferable as the mouseover and right-click would of course still be active - and should be appropriately tagged with your text.

I think this is the nearest you can get.