Been spending alot of time in Cyrodil lately and there are a few things I have picked up and seen alot of. The first thing is the type of siege used.
Pact Ballista is clearly the hands down best for any walls and doors. It hits for 2k and its reset time is stupid fast. You can get 4 shots off with Pact Ballista in the time it takes a Treb to fire once.
Using 2 Ballistas per player is the most time efficient. While one is resetting, you can fire the other and rotate between the 2. There is no downtime using this method. This also keep the enemy guessing where you are going to be since you are bouncing around every 2-3 seconds.
As far as space goes, there is no minimum range so they can be placed directly on the wall or door.
When defending against a siege is appears best to attack the siege driver and not the siege itself using a pact fire siege. The siege has 100-200k hp and the player has alot less. You can effectively keep the siege locked down by attacking the driver, therefore making him leave that siege. If the player is so intent on staying on the siege then they will die a very quick fiery death.