Walkthroughs for custom maps.

But if it assesses it as a ditch, wouldn't the computer still enter the ditch?
And in my case, it just stands there and doesn't move!

It's possible that the same principle is at work here as with the fire wall. If the computer assesses the terrain as a ditch, it might prefer not to move onto it, thinking it will take more damage than it can inflict. But this is all speculation; it needs to be tested.

It's also possible that the issue isn't with the sand, but with the broken walls. Walls 2 and 4 (counting from the top) are bugged, and the computer ignores their destruction during a siege (it doesn't move out). It might also get stuck in the gate when taking a city, thinking the gate is closed and not attacking. I've tested this, and it works. The question is whether it will enter them or ignore them if the computer attacks us and breaks through these walls.