Open Cities Reborn takes the concept of Open Cities and steps it up to the next level. The project redesigns each city covered by the original Open Cities in such a way that would make more sense strategically. The castles become a more central focus of each city's layout, with housing, commerce, and the cathedrals spread out around them in more distinct ways. When possible, concept art provided by Bethesda was followed as closely as space restrictions would allow. Where none was provided or where it was not practical to follow, creative license was used instead. City walls in several cities have been modified to suit the location. For example, Bruma now uses wooden palisades, Cheydinhal has lower brick walls that reflect Dunmer influence, and the city part of Bravil has no walls at all instead relying on their island nature. This has the added bonus that each city is now more visible as you approach. Rather than large imposing walls that block the entire view, you can see buildings on the other side along with most of the trees and bushes.
To avoid any serious compatibility issues, and because lore calls for it to remain fortified, Kvatch is not touched in any way.
The Imperial City will also remain in the layout it now has since strategically speaking, it's already built the way it should be. Although there is room in the concept art for some somewhat radical adjustments to be made. That may or may not come at a later time.
To get the most out of these changes, it is highly recommended to download Animated Window Lighting System and Chimneys. This is the evolution of Texian's Window Lighting System and is fully compatible with Open Cities Reborn.
To go with that, and provide LOD for all the new goodies, I also recommend: Really AEVWD. Just be aware that there's a bit of a performance hit in activating all the extra LOD that Open Cities will provide that's directly proportional to the power of your PC, and how many other mods you have installed.