Invisible Walls
Invisible wall in hell and holes in the catacombs are actually quite common. What’s more is that they can be recognized once you know how to identify them.

These walls are all over the places. You have probably been detoured by them often and not realize it. This happens when you are moving forward hugging the walls, you(the program) would walk around them automatically. Even monsters have to walk around them - which in turn make them tactically good fighting grounds.

Arrows or spells don’t go through these walls either. As a matter of fact, you cannot even see the monster or partner on the other side of the invisible walls. The one exception being.... Lightning or CL, they can go through some (but not all) invisible walls like the one on the left. Note- the CL gap is where the wall is.

The invisible walls are randomly generated every game. Although some of them are undetectable until you bump up against one, most of them can be anticipated. If there is a shish kebab on the wall, there’s a good chance an invisible wall exists at or a couple of squares from that location. Some shish kebabs near a wall can also create invisible walls as the one in the first animation.

However, I have not come across any invisible walls other than in the Y axis orientation (bottom-left to top-right screen) as shown in all the pix. There are at least four consistent invisible walls in every game though, one is located right outside of Diablo’s chamber as the above picture shows, two inside and on top of the maze, and one outside on the bottom of the maze.

Using Invisible Walls
Here are a couple of good examples of how to use invisible walls to your advantage whether you are swarmed or not. The one on the right shows me swarmed by a pack of BKs and a strayed witch but only 2 can get at me at the same time. The invisible wall prevented the approach of the other knights, even the Bloodstars stopped short splashing on the wall.

Invisible Walls in the Butcher's Room

First discovered by Cyrene. The gaps in the lightning strands (both ways) are the invisible walls. When standing at that spot, the Butcher will just pace back and forth along the x (east to west) axis.
The weird thing is, the Butcher will always stand still after I cast a Firewall on him.

Back to the D filez
Continue to D filez III - Holes in the Catacombs and Hell, and staircase, umm, a case of stairs ;p