You need engineers and prefects everywhere. Same thing with engineers, but for risks of buildings collapsing. But, when a prefect walks near, the risks are resetted to zero. Here's an example: As time go by, risks of fire go up. However, shipments and market traders WILL go through ! What does it mean ? It means that your doctors, your priests, your teachers, your tax collectors, your prefects, your engineers, et caetera, which are useless for anything that isn't housing (except the prefects and engineers), won't go through the gate. Units do not go through unless they actually need to go through. It's an absolutely essential building to control the pathfinding of the units in the game. The Gatehouse is not to be used only on walls, and it's not just a wall that you can walk through. You can also check out videos from Gamerzakh about it.
You can learn more in the github page of the mod:
You can buy a digital copy from GOG or Steam, or you can use an original CD-ROM version. Julius will not run without the original Caesar 3 files. "Julius is a fully working open-source version of Caesar 3, with the same logic as the original, but with some UI enhancements, that can be played on multiple platforms. It took 10 years for this project to reach completion, a MASSIVE undertaking by Bianca van Schaik, a god amongst men. The goal of Julius is to remake the game from scratch so that it is possible to run it on current computers and into the future, with some UI enhancements. Many parts of this guide has been made OBSOLETE thanks to the Julius & Augustus open-source projects. I never play Caesar III without Augustus, trust me, you want to read this!