I've been thinking about it deeply, and... I wanna start a new discussion (translation: don't shoot me)
The most intuitive position to place a fortress is the middle. It intersects the most points, it lets the army engage the most points. But what if I told you that such a decision is WRONG?
The best position for a fortress is not in the middle, but on "one of the wings," aka on the sides of the battle. THE REASON is: If you place a fortress in the center, that place becomes inaccessible for the opponent. So, they can go to the left or to the right. To respond to an attack on the right, you have to go through left to center to right. To respond to an attack on the left, you have to attack from the right to the center and to the left. With a mobile force? I can forgive this oversight.
Furthermore, to attack your base, an opponent will have to go through their base to the left to your base, ignoring your army. Or they have a second attack point! Through their base to the right to your base. This gives the opponent 2 fronts to attack you from, with a delayed response time, that can let the opponent attack 3 parts of your base: The left, the center, the right!
But let's think differently. A fortress on the left corner! Obviously, two fortresses are too expensive, so we will ignore that option and focus on just the fortress on the left. The opponent has two fronts to attack, ignoring your fort: Center and the right wing. To reach the center, your army can go directly from the right wing. To reach your right wing, the army can move from the center to the right. And if the fortress is built with enough expertese, the opponent will avoid going through the fort, and it won't be a major expense.
But let's assume that the opponent got to your base. Through the right, they can only attack to the right, or be cut off by your army.
Through the center, they can only attack to the left and risk getting squished by the fort. Engaging your base on the center means fighting your army guaranteed.
TLDR: Place a fort on the wings aka sides of the battle, place your army in the middle to engage in center and the right.
Visualization:
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Inspired by (the failure of) this video:
