From a security standpoint, if we’re just talking zombies and assuming that the zombies can’t climb, maybe a pre-cannon-era castle or similar fortification. I think that most of the things that obsoleted historic fortifications wouldn’t really apply to zombies.

I don’t mean one of the castle-themed buildings, like a folly. But something where you don’t have any ground-level windows aside from slits, and probably has walls around it.

You may have defense-in-depth (multiple layers of walls or building structure, with the building and walls designed to permit a retreat to an inner area if an outer area is compromised).

Cisterns for freshwater storage are likely already present (though I’ve no idea what condition they might be in) so you don’t need to get ahold of more storage.

Ample room for storage.

I guess the major issue might be the degree to which any fortifications might have been converted for public-access use. I don’t know how many gates and porticullises might have been removed or disabled over the years because they aren’t really necessary if the fortification is essentially a museum.