Its a successful attempt to improve RTW's gameplay. It uses the mundus magnus map and meddles with all of the original factions making their unit rosters and locations historically accurate. It uses cultures the same way religions are used. conquering a city that has a different culture than your faction will have lower public order status but will allow you to recruit some (un-re-trainable) units that belong to that culture. though, just like a religion, the indigenous culture will slowly be replaced. Traits are seriously improved to behave more dynamic. Trait come and go depending on the characters situation such as age. Being too young or old has its plus and minus. Also different cultures have their own separate order of traits instead of the generic roman traits that appeared in all factions in RTW. cities take even longer to control as you first need to build the various garrison buildings in order to assert full dominance (and then full access to the cities taxes and building tree and unit roster). However cities become harder to control the farther they become from your capital so you can instead turn them into allies of them empire. this gives you limited control but they're happier that way. Armies are also way more expensive to recruit and maintain. This limits the amount of armies (for those that can't really afford them) and therefore makes battles more significant.
for example: I am pontus fighting the Seleucid empire. The seleucids brought a big army against me and I had to desperately pool all of my resources to get the mercenaries and some of my tier one units to rally against their better trained army. I won with a heroic victory and they lost their military might in the region. That one battle now means I can take all of their cities in western turkey with out any problem.
You can find the download
here. just click on the name Extended Cultures 4 1.0 that should be at the top of the page in blue and the download should start.
I've really enjoyed XC so far and their may have been things I missed in my description but I hope this helps.