In the real world, if your supply lines are cut, you will run out of food, water, ammunition, and eventually become ineffective.
In this game, if your opponent spams fortifications, but you have map control, its better to ignore his fortifications and go for his villagers. Unless of course the fortifications are well placed. Anyways, what if there was another method to deal with fortifications? As in, if you lay siege to them, they would weaken and die? Enter the supply lines concept.
Every time a worker returns to a towncenter with food, they drop off a second resource called supplies. Every few minutes, each tower and keep sends a request to the nearest towncenter for supplies, and a donkey is dispatched with supplies. For the player, the donkeys are not controllable. However, they can be attacked by enemies.
Once a donkey with supplies arrives at the tower or keep that called it, the building is resupplied, and can go another 3-5 minutes without requesting new supplies. If the building misses its supply, it will call for a second supply in 3 minutes. If it misses 3 supplies in a row (10 minutes) the building will become capturable, simply by having any unit of any faction garrison it.
Special exception: Mongols do not use supplies.