Glad you found a way to get around it then. Usually the game does bar your getting back into the room once the spiders are in there. Your invisible walls I remember running into a while back as well- a glitch, I think, with the hitbox of the spiders. Occasionally- just occasionally- the game engine used in the Alone in the Dark games seems to get confused about where the hitboxes are for certain objects, as I remember having somehow managed to clip through one of the hedges in the maze at the beginning of Alone in the Dark 2 once, resulting in my not being able to progress since the hitbox that would move you to another screen didn't recognize me anymore.

Yeah, the collisions suck in pretty much the whole trilogy. Just today I went through a wall in the first Alone in the Dark into the bathroom with the bathtub monster. This didn't do any harm, especially since I'm saving all the time but still such events are quite an annoyance. In my memory the engine was somewhat more reliable.