If you've actually cleared the north fields before you enter the city, which includes waiting for the handful (3-5) of vehicles to exit the city and join the fight in the north fields as well as waiting for the vehicles that spawn in the river west of the town to either a) be destroyed early because you spotted them when they spawned, b) climb up into the north fields to fight, or c) climb up and enter the city then move through it and exit the city to join the fight in the north fields, then it really doesn't matter which route you use to enter the city because there will only be a handful enemies left and none of them have LOS to any of the entrances. One is usually hiding near-ish the yellow circle on my map, one or two are usually hiding or stuck slightly south but mostly due west of the fountain in the center of the square, and then sometimes there's a third group of one or two in the buildings SSE of the center of the square.
It is very important to not try to enter the city from the north until the last group of enemies (usually 4 of them) spawn along the north edge of the map. If you move in too soon, those 4 enemies will spawn and attack you from the north right about the same time that the handful of enemies exits the city on the dirt road and you'll be trapped in a cross fire.