Open Cities Skyrim is a comprehensive project that aims to bring back at least part of the feel of Morrowind - specifically how most of the cities were a natural part of the world and you could just walk in without loading screens. The added immersion and realism goes a long way, probably a lot more than you might think. Ever wanted to ride your horse into the center of Riften? Well now you can. Need to get some help from the city guards to vanquish a foe? Run toward the gates and the guards will do their duty while you can seek safety within the walls. The town guards make for some very interesting interactions this way. This will also make running dragon battles that much more interesting since the flow of the battle won't be interrupted by the loading screen and all of the city guards can be involved at the same time.
 
Installation Requirements

Skyrim Special Edition 1.6.1130 or greater.
Unofficial Skyrim Special Edition Patch 4.3.0 or greater.

DO NOT extract the contents of the BSA file into your Data folder as loose files, or let your mod manager perform an equivalent function on the contents of the file. Doing so will cause things to load in the wrong order and break the game. Skyrim is intended to load a BSA alongside it's ESM/ESP file. Circumventing this causes issues which I will not provide support for. If you insist on going against this advice, you will receive no support for the problems you generate as a result of this.

Installation - Wrye Bash BAIN

Drop the archive into your Skyrim Mods\Bash Installers folder, then install as usual.
Make sure the mod is active in the Wrye Bash mods tab.

Installation - Nexus Mod Manager

Use the "Download with Manager" button on the Open Cities Skyrim page at Skyrim Nexus. The installer should take care of things from there.
Then simply make sure NMM has the mod activated.

Installation - Manual

Drop the Open Cities Skyrim.esp, Open Cities Skyrim.bsa, and Open Cities Skyrim - Textures.bsa files into your Data folder.
Activate the mod using whatever management tool suits you.

Post Install Configuration

Set the mod's load order using whatever management tool you feel most comfortable with.

It may also be necessary to rebuild your Bashed Patch if you're using one.

Once the game loads, you need to allow 24+ game hours to pass for AI to fully update in each city.

You can configure three options in-game by using a book that's been placed inside the Temple of Kynareth. This book is free for you to take with you, and you can put it on a bookshel anywhere you want. It would be STRONGLY recommended that you do not destroy it in some way because the menu will be inaccessible without it.

Option 1: Oblivion Gates. The menu will now allow you to toggle them on or off as desired, on the fly, with no external patches necessary. If you have any such patches, uninstall them. They'll crash the game now.

Option 2: Gate Attendants. The gate attendants who open the city gates when you approach on horseback can be toggled on or off as desired. Yes, some people despised them enough to ask for this. Don't ask me why.

Option 3: Outdoor Lighting System. You can toggle the state of the outdoor lighting system as desired. Nobody asked for this, but I figured what the heck. I was messing with MCM anyway so why not.

These optional configuration elements will not be supported through any other mechanism.

Uninstalling Open Cities Skyrim
 
Due to the nature of Skyrim mods, especially those adding scripts, it is strongly discouraged to attempt uninstalling a mod from your active game. Any mod that is more than pure mesh or texture replacements has the possibility to leave behind permanent changes to your save that you may not want. This is not something modders can correct for. It is how the game was designed by Bethesda. No support will be provided for anyone who uninstalls this mod from an active game and continues on with that save.

Load Order

Open Cities Skyrim should be loaded as late as possible in order to preserve changes to navmeshes and other exterior city data.
 
LOOT should be used for optimal placement.

Built-in Compatibility

Falskaar             - Jalamar's AI markers will be moved into the proper places in Riften.

Compatibility Patches

Skyrim Sewers 4        - Available in the patch archive at the Open Cities Skyrim download sites (except Steam).
Drinking Fountains of Skyrim - Available in the patch archive at the Open Cities Skyrim download sites (except Steam).
Dawn of Skyrim Collection
Immersive Citizens - AI Overhaul - Included in Shurah's installer.

Compatibility

Mods which strictly replace vanilla textures or the vanilla meshes are 100% compatible.

Expanded Towns and Cities has been confirmed to be 100% compatible since it does not affect the 5 walled cities.

Generally speaking, any mod which adds things to the closed cities will be incompatible.

City mods that add some details to the open worldspace will be partially compatible.

Alternate Start: Live Another Life - Fully compatible. No patches needed.

The Last Defeat of the Dragon Cult - The mod hasn't been updated since March 2012. It's navmeshes are partially corrupted and must be repaired.

More Dynamic Shadows & Striping Fix - Numerous dirty edits that cause some interiors to cease functioning.

Mods which edit the following scripts will not be compatible unless patched:

QF_DialogueWhiterunArgueScene_00037CC2
SF_MS11OpeningCrimeScene_000206AE
SF_MS11SecondMurderCrimeScene_0002422C
SF_DA10StartScene_00023C43

Known Issues

The Civil War battles which take place in Whiterun, Solitude, and Windhelm will be fought within the closed city worldspaces. This had to be done due to complications that were not possible to overcome with the scripting involved in the war and how it relates to several other quests. Expect load door transitions when a city battle is underway.

Troubleshooting Problems

If you have a problem with an Open Cities mod and wish to report it, I'd appreciate as much information as possible, preferably in the form of a screenshot with console data showing if it's something that can be screened. The FormID of the affected object would help greatly to narrow down any problems that might result. Post this information along with your load order. Reports with less than detailed information may result in delays in getting it fixed, or result in being ignored entirely. It helps everyone to provide details. I am especially interested in verifiable conflicts, as I wish for Open Cities Skyrim to be as conflict-free as is humanly possible given the circumstances. If the situation requires a patch, I can only work something out if I know what to look for.

The doors inside Warmaidens (in Whiterun) are missing, what have you done with them!

The More Dynamic Shadows & Striping Fix mod makes a bunch of dirty edits in Warmaidens that will cause the doors to disappear. This mod has several other problem locations as well and it isn't recommended to be used in general until the author fixes the problem.

Locations of AI Doors

The AI doors are entry ways into the closed wordspaces should it become necessary. Not everything that is placed in a city by a mod is worth the time it takes to create a patch. Mods that drop items for a one-time pickup are a good example of this. It's much easier to just go through the door, get what you need, then exit the city as normal.

Riften: East of the main city gate, next to the alley gate.
Markarth: Enter the city gate, turn immediately to your right. The small rock in the grass patch next to the ivy.
Whiterun #1: East side of the bridge into the city, right next to the brazier that is closest to the guard shack.
Whiterun #2: North side of Jorvaskr, on the side of the path up to the Skyforge.
Solitude #1: In the rock pile between the city gate and the executioner's platform.
Solitude #2: In the rock pile across from the Bard's College, immediately to the south of the manhole cover.
Windhelm: As you enter the city gate, turn immediately to the right, then immediately right again. It's in the bushes right next to the wall.

Performance and FPS

Open Cities Skyrim should not produce a significant change in your frame rates and performance. Due to the liberal use of occlusion planes in the mod, the game will not render anything on the opposite side of the city walls in any given location. So your viewing content will be limited to roughly what you'd see if you were in the closed city worldspaces. The closed city worldspace system was NOT devised by Bethesda to improve frame rates. It was devised to conserve system memory on the XBox 360 and PS3. You're playing on a PC. Or, at least legally speaking you ought to be anyway.

Anyone who insists that this mod is a framerate destroyer is already experiencing other problems with their game. Do keep in mind, Skyrim is not one of those silly shooters that requires 60fps at all times in order to be playable. Dropping from 60 to 55 would go absolutely unnoticed. Even playing with 30-40 frames would go unnoticed to those folks who keep those dumb little indicators running while they play.

If you're seeing a massive frame rate drop, point the finger somewhere else. This mod is absolutely not the cause of your problems. I know this may seem harsh, but this sort of disinformation was rampant with the Oblivion mod and wasn't true then either and I absolutely will not stand for this sort of disinformation being spread now either.

Credits

Throttlekitty - Help with the Riften city gate collision sizing.
BassEX - Corrections made to the Markarth dam mesh.
Hannaise - Oblivion Gates in Skyrim
jonwd7 - Fixing the collision on 6 Whiterun wall pieces without normal collision height.
pihwht@Nexus - For having the patience and dedication to dig deep into the civil war and ferret out problems causing it to fail in Solitude.
Dwip - For numerous improved meshes at the various city gates. No more crappy fake collisions!
nonoodles - Riften southeast gate mesh plus the animated dock gates.
Nico coiN - Cut back version of the Whiterun drawbridge mesh as well as numerous other cleanups and fixes for the mod.

Disclaimer

This mod is not made, guaranteed, or supported by Microsoft, Zenimax, Bethesda Game Studios, or any of their affiliates.