As I said before, my problem with garrison composition is that medium infantry, which is what the levy spearmen are, simply cannot hold very long against heavy or very heavy infantry. They also rout fairly quickly when pressure is applied to them, making for relatively quick siege battles. My assault on Roma took about 20 minutes and that was with the pre-assault bombardment and the post-battle victory parade through the city, so the actual battle was about 10 minutes.

A compromise for this would be to give the faction capital much better troops, in addition to the level IV cites. Cities level III and above have walled ranged defenses like onagers, so I feel as an added defense should be my successive capture point system and the added garrison. I suppose that in non-capital cities they can be just run of the mill levies or the like, but for all the capitals, not just Roma and Carthage, they should have better units because after all, it's the seat of government and should be defended as such.