The appearance will change based on what level the city is, getting more impressive at each level up. It is a graphical mod that changes the appearance on the campaign map for faction capitals. I always use the GCCM faction capitals mod. So you can spend a max of 5 points to rare, 10 to special and no limit on core. I occasionally use the Closer to Tabletop Unit Cap Mod which gives each unit a teir and a value for that tier (core, special & rare) and enforces maximum caps for units of that teir.
They add a whole bunch of new LLs to the game as well as new hero typea, mount options and some Legendary Heroes. I'm quite partial to all of Mixu's Legendary Lord Mods.