There is plenty of precedent for hidden facilities - some of them quite large - even in famous pieces of architecture:

https://www.oddee.com/item_99554.aspx

So far, no-one has given me grief over superhero bases in my Masks stories. This should head off anyone doing so. :-)

The Vatican is famous not only for concealed nooks and crannies, but walkways high above ground.

Anyway, Happy Holidays to all!