I should point out that sieges were often rather large ordeals. If a city is several kilometres in diameter, the besieging army has to cover an immense amount of terrain, since they don't want to go near the range of the city or fort's artillery. My point is, they're not standing right in front of the fort, and their artillery might not even be in range (lest it get hit by enemy artillery). Besieging a mountain fort mostly involves blocking off paths and roads and screening the surrounding area with scouts. A garrison that sorties would have no particular advantage in assaulting a road block compared to the people defending it. On the contrary, the besiegers are the ones who get to choose their positions.
When it comes to more flat terrain, a sortie would probably be fought just outside the city. Due to the distance and the fact that the besiegers would be out of formation to begin with, there would be time for the sortie to get in formation, but they still would be silly to leave the range of friendly artillery and enter the range of enemy artillery. In other words, the sortie generally exits the city, at which point the besiegers would or wouldn't take the bait and attack. If they didn't, the sortie would take a huge risk in attacking entrenched positions with artillery without even having artillery of their own.
I guess my point is that the besiegers probably have the terrain advantage either way. But that's assuming there's terrain near the city or fort to be advantaged by. Additionally, it would make sense for the game to have a "dug in" bonus. Trenches warfare wasn't a thing, so "dug" might be the wrong word, but any general would probably realise what could be done to improve the defensiveness of whatever location their troops were stationed.