There is two KH in my town, one has just one floor with massive one way view glass side entrance, visible from last rows, and it is not locked during the meetings.
Other one has two floors, 1st with apartment for CO, lavatory, desk for literature, office, etc. and 2nd floor with hall, and entrance is locked during the meetings, the ringer is LED blink visible from last rows. There is also an emergency one way exit on the back - some older folks use those when they are late and knock like hell during opening prayer :)
I think the most of this security measures is demanded by insurance policy, rather than some freaking cult paranoia.