My attempt at Mediolanum keeps failing due to my city devolving housing. It occurs at the same time in the game. I am not sure what is happening.
I posted some questions on the C3 forum and Brugle was kind enough to answer me. I tried some of his suggestions without success. He suggested I submit my city for review, so I am.
Thank you for this website. It has really helped me understand C3 better.
I think the problems here can be attributed to building much too fast, before you have workers. At the save you are 572 workers short. You have buildings in the middle of your housing block that are not needed, such as academy, schools and libraries. You have unstaffed temples and are likely to be cursed by the gods sooner or later, if you have them turned on. An important rule in all games of this type is to only build what you have the workers for. You have to start slowly and prioritise. Get people in as quickly as possible but only build what is essential.
You only need 25 prosperity in Mediolanum, so I'd stick to large tents in several smallish blocks (I use 16x5 in missions that only require lower housing levels but there are lots of good block designs in the downloads section) until you have your industry and trade up and running. Don't pay attention to Caesar's advice in the tutorials about food being first priority. Only when trade is maxed and you are confident of being able to defend the invasions should you provide your people food and services, other than clinics, engineers and sufficient religion to avoid being cursed. Prefects are not needed in this scenario. Only up to small casa is needed on this mission. It is pretty much the game's military tutorial.
In short, more housing in smaller self-contained blocks of large tent level at first. Build only what you need and do not commit workers you do not have.
Also you've got your forts far from where the invasions occur, occupying farmland you'll eventually need to meet the population requirement. A clue is given in the briefing. They come across the alps, which is to the north.