First Prosperity is a bit special as the way it is calculated it will fluctuate a lot depending on your current city circumstance (made even worse if you're not planning ahead your city blocks very well
) when houses devolve or improve or if you have too many low quality "tents", how things are calculated for Prosperity is explained there :
http://caesar3.heavengames.com/cgi-bin/caeforumscgi/display.cgi?action=st&fn=2&tn=1489&st=1#post2it only change at every start of a year, so make sure you're not losing your housing bonus just at the end of the year and don't throw yourself into big building to kill your end of the year benefices).
Now my problem with Lugdunum is the distance and spacing, the best open space is located extremely far from the only sources of farming (due to there's only 1 entrance to that farm-friendly area), to bring food your guys will have to carry it all over the map before reaching the granary in that nice open space.
Everywhere else there are rocks, natives huts and lots of things that will make it a pain to make some nice housing.
But it's in those places that it's important to make it the nicest as they're the only location in which they're not risking house devolution like the away city blocks that loves to devolve a bit before getting the next batch of food (and hurt your Prosperity a lot if it happens just near the end of the year).
Augustus helped me a lot with the new extra large temples that are built in steps and require tons of material (a feature ported from Pharaoh/Cleopatra) as when i finally managed to complete the Ceres huge temple it allowed my food transporter to move faster so i finally got the away large population block to stop devolving so easily.
Other problem, when your housing improve above the largest insulae into the villa stages , it's like in DF when you have nobles : they don't work, so you suddenly have huge deficiency in your working force and some facilities aren't at their best anymore, failing to produce as fast the items required to keep some house to devolve.
So you build more house and takes care to not have them improved enough to become villa, but it require food for those to not devolve, and if you're not building your perfect house blocks you're going to have a kind of circular problem hitting you all the time. And in the same time you need to have villas too due to how the Prosperity cap is calculated. It's just hell
Fortunately, Augustus again is helpfull with the optional "Global labour pool" features that was ported from the Zeus/Poseidon games and allow to use worker on locations without requiring them to live very close like in default Caesar3 , i think without that feature my dodgy city would have never hit the Prosperity requirement for that map despite my best efforts
Oh, and Caesar3 population age, when they reach a certain age and retire they don't work anymore, but they still need feeding and etc.. as they're still part of your population, and you can then get horrible hit on your work force and can only count on future migrant by increasing your city housing (and dive deeper into the Prosperity management hell) , i think the feature is actually an attempt to make a time limit to the mission, as it makes everything much harder if you take too much your time as you work force is hit very badly if you play very long.
Fortunately Augustus has an optional feature to get the citizen work 10 more years before retiring, i think it helped greatly my slow paced attempts at getting my Prosperity reaching the requirement.