How does a battle happen against an entrenched foe? Meade was not defending the shoe factory in Gettysburg or the farm of a freed slave. Lee wasn't assaulting Washington D.C., so why was there a battle?
First, let's set down the facts. A Division in line has a front facing the enemy of about one mile. If you were to spread the Division any thinner, they would more easily crack in an assault. Line the men up in deeper formations and they would be a prime target for cannonade. I posted in another thread how level 4 entrenchment stopped only around a third of the hits a unit would take. A breastwork would stop easily twice that, so the best protection a unit can get from mere "entrenchment" is pretty much a gift of the terrain. Entrenchment in game terms is not a line of WWI trenches. That would be a stockade or fort. You benefit more from a farm fence or a railroad cut than from a shovel and an axe.
So where is my army entrenched? If they are in a structure in a region, then that is an easy question to answer. But what if they are entrenched outside any structure in a region with a river? Then according to the game, they are situated to fire at passing ships along the river. However, they cover any structure/city anywhere in the region from siege, too. They also block any force moving across any portion of the river. Except that a river border of a region is many miles long. It would take most of a combatant nation's Divisions to entrench along its entire length...and that is just one region. Similarly, an army is not entrenched along every one of the region's other borders, as this would be even more impossible. So, again, where is my army entrenched?
An army cannot be entrenched along the entire length of a region's river(s). It cannot be entrenched along every border of a region, either. It cannot be entrenched outside of a city and along a river and along another border at the same time. These are simply impossible. I would say that an entrenched army is somewhere in a region in bivouac. When an enemy moves to enter the region, then scouts tell the overall commander. He counter-marches inside the region to meet the enemy. The "time to entrench" factor reflects that he knows the region better and gets there "first with the most". As such, he defends on "good ground" so that his troops get the benefit of level X entrenchment. The ground itself may only be a few square miles and is otherwise insignificant. Perhaps some other units MTSG as well. The enemy cannot leave his force on their line of communications and must defeat him...or withdraw from the region. That's how a battle happens against an entrenched foe.