For a house of apartment........ three inches from the opening side drill a 1/2 inch hole on the floor and then place a 1/2 inch round piece of metal in the same.
This will allowed you to open the door those three inches in safefty in order to find out who is there.
The only way to force it open would be by forcing off the whole frame......better with a metal door.

"If you are ready you are not"... Ponce