From: Houston, TX
So I can assume that for a large infantry corps that always goes from D->E->F in three turns, the entrenchment level just doesn't matter. But for smaller units which have more difficult times moving up that latter, entrenchment levels are quite important. Is this correct?
It might even skip one of the D or E steps. I don't think unit size matters in digging in. If the scenario has a reduced entrenchement factor, though, the effects may become more significant.
When I hover over a terrain hex several pieces of information show up, such as the terrain type, entrenchment level, etc. Sometimes, when hovering over urban areas, fortified lines etc, I'll also see values such as "AT=800%, AP=500%". What do these mean, and how do they work in conjunction with fortification status, etc?
You're talking about the Tool Tips. I don't use them, so take that into consideration. But I assume the values are the terrain benefits. So the figures above would equate to a x8 benefit against AT and a x5 benefit against AP. As of 3.4, terrain benefits combine with deployment benefits as the squareroot of the sum of the squares. See section IV. 11 of the 3.4 part of the What's New document.