There is several way to build a wall and gate.
Age of empire 4 do not use wall and gate at their full potential. 
Some stone wall are tiny like a wood wall and their gate is only a wood door.
Other wall are empty inside, allowing to place buildind inside and the gate door is made of a herse and a wood door.
Some stone wall and stone gate , hide deep inside them the old wood wall structure and the old wood gate structure.
Some stone wall or stone gate are build on a cliff side making enemy waste presious siege munitions and time, on attacking a cliff that will never fall.
Some stone gate are upgraded whit time:
- Hadding conterwait to open herse fast and conter attack.
- Hadding a hamster weel to open heavier gate.
- Hadding a second layer of gate.
- Hadding a oil drop off and fire arrows, to conter ram.
Also, stone gate are made to be clostrophobic, to limit the quantity of enemy that enter the broken door.
But the main city gate is ofthen wide, to let alot of people in and out, fast.
Side note:
- Ram is not suposed to break the intire gate fast, its suposed to break the door first. XD