There is a scene prop named “barrier” which is a very narrow 1-way invisible wall which blocks players from moving through from a certain direction.
Can’t remember exactly how it is in BL editor (though a lot is the same from Warband), but there’s many sizes and shapes of barriers, and also a “horse barrier” which blocks all horses, (obv. including cavalry). It’s used for example on stairs and other steep climbs not intended for horses to cross.
A usual way unreachable parts of a map are made is to put for example, barrels and carts on a city map’s alley to block it off. Of course you either have to stack an unnatural mountain of barrels to stop a player from actually climbing over them, or simply put a barrier right in front or behind them to block the area off.
In BL I think all props have collision where you’d expect them to. So no invis walls, except an invis wall prop called “barrier”. If the barrier prop isn’t put in, you can pretty much climb up any hill and jump over any wall that’s not too tall.