The first two that come to mind are a training yard to improve the quality of recruits from cities up to tier 3 (obviously would need to add recruiting from cities...) and a school (villages get schools but no cities? Really?) to increase prosperity. Like villages, investing in these up to a certain limit would be allowed (unless the lord hates the PC) and after that permission would be required. The training yard would add soldiers to the city's defense as well. After that, I don't know. My brain is melted. Maybe no more are needed since higher quality goods are taken care of in the Prosperity section at the top and these would take care of better troops and EVEN MORE tax revenue. What else do you need from a city after that?