Why?

Valtheim Towers is not a particularly special place in vanilla Skyrim. You have two towers and a bridge, with just a sprinkle of bandits, one of whom will harass you for a toll and get the rest of the bandits killed for her trouble. But look at where Valtheim is, what it is. Valtheim is located right on top of the main road from Whiterun to Eastmarch, in the middle of a narrow valley between two mountains. By all rights, a proper fortress where Valtheim is should be able to control the ability to travel from Whiterun to Eastmarch - and in the middle of a civil war, that control would be invaluable. It always bothered me that vanilla Valtheim was so much less than what it could be - so I changed it.

What?

Tactical Valtheim is a redesign of Valtheim Towers into Valtheim Keep. I've built on and around the original Valtheim, trying to make it into an actual fortress while maintaining something like the original atmosphere and style. Here's what I add to Valtheim:

• Walls and arches: the road now passes straight through Valtheim's walls. The river is walled off, as is going around the fort, leaving the only open path through Valtheim. Travelling through Valtheim now puts you right in a killbox, meaning that if you control the fort, you control the road.
• More towers, bridges, and overlooks: in essence, more spots to put archers, making Valtheim more dangerous.
• More people and patrols: Valtheim is a much bigger place. It would be ridiculous if I doubled the amount of Valtheim to guard and didn't double the number of guards and how much of the fortress they guard. There are now more bandits, and patrols that cover the entirety of the new fort.
• Garrison the fort with your allies: more on this in a moment.
• A unique enchanted tower shield: as a reward for clearing Valtheim, you can find the Shield of Valtheim in the fort. It's a fairly powerful shield, with a strong enchantment. To compensate for this, it's also hidden - I'm not just giving you a powerful artifact in exchange for one of the first bandit camps you see. There are other enchanted "weapons" lying around, but they're garbage, so don't bother looking for them.
  

Claiming Valtheim

After you clear Valtheim, you can claim it for Whiterun, the Empire, or the Stormcloaks. Since it's on the Whiterun-Eastmarch border, it has an important position that all three factions would care about. Claiming Valtheim Keep for any faction will result in Valtheim being garrisoned by that faction instead of bandits, will add a generic blacksmith by the new forge, and grant you access to the quarters at the top of the northern tower as a small player home with a minor gold reward. The quarters will be renovated post claiming the fort; please note that the quarters in the video are only the unupdated vanilla version. Despite being in the open, the quarters also have a heat source added so you won't freeze to death if you're using Frostfall.

To claim Valtheim, simply clear Valtheim of enemies, find one of the four eye-level green banners, and activate it to raise the banner for the faction of your choice. The control will be updated after 5 real-time minutes, 1 in-game day, or once Valtheim has unloaded.

Compatibility

Probably not compatible with anything that changes Valtheim. 3 guesses why. On the other hand, I don't know of any other mods that change Valtheim, so this probably isn't an issue.
Apparently not compatible with Druid's Den.
This mod has been integrated into Invasion of Skyrim, so don't use this mod alongside that one. Bad things will probably happen.
There's now a Requiem patch. If you use Requiem and don't use the patch, then there will be bandits left over after you've claimed Valtheim and that will be your fault.
There's a patch for Enhanced Landscapes.

Credits

TheMalfazar for a shield from Mega Shields
Artisanix for the painting in the tower
/u/TRAWZ, /u/Valyn_Kt, /u/RolandTEC from /r/skyrimmods for the screenshots - my laptop can't into graphics, and they were kind enough to provide the images I use for this mod. You guys are the best.
Thanks to Hodilton for the video.