You can think of each Stronghold Turn as being an extra reward for any full completion of a Quest (Main Quest, Side Quest, or Task), whenever a Quest is marked off as finished in the Quest Journal.
How to Have a Good Stronghold Experience
Since Stronghold events only occur on Stronghold Turns, it makes sense to try to avoid turning in Quests before acquiring your Stronghold. However, there are some Quests that are required in order to get the Stronghold itself.
Once you get the Stronghold, rationing Quest turn-in's allows you to save up Stronghold Turns for when your Stronghold has higher Prestige and Security ratings (meaning you get more favorable events per Stronghold Turn). This means focusing on Upgrades before turning in most Quests.
When you start an Upgrade, simply leave for a different map and come back until the upgrade is complete, then select another upgrade. A simple method is traveling back and forth from, say, the Caed Nua map to Gilded Vale after choosing each upgrade, until the upgrade is finished, and then selecting the next upgrade and repeating.
However, upgrades require money. The best source of money is in the Endless Paths below Caed Nua -- containers hold an enormous amount of coins. Ideally, you would loot every container without fighting any monsters. Unfortunately, the enemies get progressively tougher the deeper you go. At some point, you will be forced to do some Quest turn-in's just to level up your party -- when that happens, I recommend completing as many Prestige- and Security-boosting upgrades as possible first, before turning in any Quests. The boosted Prestige & Security scores makes it more likely to get a favorable Stronghold event with each Quest turn-in.
You must complete the main quest in Heritage Hill before you can completely conquer the Endless Paths. Once you have conquered Caed Nua, speak with the Steward for a Quest turn-in and a significant boost to both Prestige & Security (+8 each).
Here is my recommended order of doing upgrades:
Eastern Barbican (required) - the only upgrade you can do at first; costs no money and is finished instantly
At this point, decide whether you want to invest any time and energy into the Stronghold: maintaining the Stronghold changes the way you play the game significantly, in that you have to treat Quest turn-in's as Stronghold Turns and ration them accordingly. This can significantly detract from an RP experience, as you must meta-game. You will also need enormous amounts of money, which would otherwise go into enchanting your weapons and armor, or learning new spells for your wizards.
If you decide to continue with the Stronghold, then here is the rest of my recommendations on the order of upgrades:
Brighthollow Manor - the only upgrade you can do after Eastern Barbican; allows you to rest for free anywhere on the Caed Nua map except in the Endless Paths (simply click the Bed icon at the bottom of the screen)
Main Keep - unlocks other upgrades; gives a nice boost to both Prestige & Security (allowing you to collect more taxes)
Western Barbican - unlocks more upgrades; also a cheap way to boost Security
Curio Shop - gives you free Creature Parts every Stronghold Turn
Bailey - unlocks more upgrades; also a cheap way to boost Prestige
Botanical Garden - gives you free Herbs every Stronghold Turn
Warden's Lodge - allows you to get bounties for lots of money; note that each time you kill a bounty target, you trigger a Stronghold Turn, and then you trigger another Stronghold Turn when you turn in the bounty to the Warden.
Afterwards, try to keep Prestige & Security at about the same levels as you choose upgrades. Focusing just on Security makes favorable events almost never happen, which can quickly make you resent your Stronghold. Keeping the two at about the same level gives you a good mix between favorable and unfavorable events.
Here are upgrades I recommend getting last:
Barracks - You generally won't need hirelings as long as you keep your character level low. Otherwise, they can be very helpful during Attacks at higher character levels, when the attackers are much more powerful. Note that hirelings are a constant drain on your income per *game day* rather than Stronghold Turn, and are generally not worth it if your income is limited and you still have Stronghold upgrades left. Best advice is to keep character level low rather than sink money into hirelings. The Prestige & Security bonuses you gain from hirelings do not affect the amount of taxes that you collect or that get stolen by bandits; in fact, Prestige & Security do not affect your taxes, period.
Dungeons - if you are rationing Quest turn-in's, then it's unlikely you will ever get a chance to use the Dungeons until you start doing Quest turn-in's frequently again.
Merchant Stall - you can already sell things at your Curio Shop.
Towers - cheap way to boost Prestige & Security.
However, once I built Towers, Stronghold Attacks started happening! (??)
If Security is low, then best to keep within 3 days' travel distance from Stronghold (so far, minimum warning time for Attacks has been 3 days).
Is it that building Towers triggers Stronghold attacks, or once Prestige has reached a certain threshold (10?) the attacks come? In that case, upgrades that raise Security without raising Prestige are best in the beginning? I think I tried this, and that doesn't work either -- attacks just come no matter what.
Prestige & Security
Higher Security allows visiting Hirelings to safely arrive at your Stronghold.
However, I've found Security has a seemingly random relationship with what percentage of taxes get stolen by bandits. It does not seem to consistently affect the percentage stolen.
With 3 Prestige & 2 Security: 48 cp lost to bandits, 626 cp arrived safely. Loss = 7.12%
With 20 Prestige & 22 Security: 28 cp lost to bandits, 828 cp arrived safely. Loss = 3.27%
With 28 Prestige & 27 Security: 115 cp lost, 506 arrived. Loss = 18.5%
With 31 Prestige & 30 Security: 435 cp lost, 880 arrived. Loss = 33.1%
With 34 Prestige & 30 Security: 273 cp lost, 440 arrived. Loss = 38.3%
With 48 Prestige & 46 Security: 678 cp lost, 247 arrived. Loss = 73.3%
With 48 Prestige & 46 Security: 674 cp lost, 181 arrived. Loss = 78.8%
With 48 Prestige & 46 Security: 685 cp lost, 253 arrived. Loss = 73.0%
With 48 Prestige & 46 Security: 623 cp lost, 229 arrived. Loss = 73.1%
(Changing Prestige & Security scores via Hirelings:)
With 44 Prestige & 38 Security: 696 cp lost, 165 arrived. Loss = 80.8%
With 44 Prestige & 46 Security: 655 cp lost, 246 arrived. Loss = 72.6%
Summary: the amount of taxes collected does not scale with the main character's level, nor with the Prestige score.
Prestige seems to not have any connection with how much money is collected in taxes every 5 Stronghold Turns: 856 cp taxes in total with just 20 Prestige, while only 713 cp taxes in total with 34 Prestige. Security also seems to not have any connection with how much money is stolen by bandits. For instance, just 7.12% of taxes was stolen with Security score 2, while 78.8% was stolen with Security level 46.
(Note: Test if it's the ratio of Prestige to Security that counts instead?)
Attacks on the Stronghold
I've found that the level of the creatures attacking the Stronghold scales exponentially with the main character's level. If you stay at a low level when you're just starting to upgrade your Stronghold, you can keep the Attacks weak enough that your party alone can handle them. Again, this means withholding any Quest turn-in's until you've upgraded your Stronghold as much as you can between each Quest turn-in.
Stronghold Attacks are rolled for each game day.
If you get a Bad Visitor to the Stronghold upon entering a new map, you can re-load the game to the Autosave (if it's been turned on) of the old map where you came from -- if you don't have an Autosave, then you can try hoping you have a Save that is not too far from when you left the previous map. Once you have that Autosave loaded, save it in a new Save slot (or you can skip this step and hope the game doesn't overwrite the Autosave you want). Then completely exit the game, restart Pillars, and load that Save. Now travel to the new map again. You should now get a different Visitor. (Pillars keeps giving you the exact same visitor each time you reload unless you clear its buffer by exiting the game completely.)
I am completely sick of the Pillars of Eternity Stronghold and refuse to do any more work on it. If you have any comments or suggestions about this post, please post directly here. Unfortunately, there's no way to upvote / downvote useful comments on the Obsidian forums like there is on Reddit.
Suggestions to the Obsidian Team on the Stronghold:
Please allow people to accumulate Stronghold Turns whenever they turn in a quest, and then allow them to choose *when* they want to spend those turns.
Why does anyone want to play a game that takes control out of the hands of the players? That's the definition of an unenjoyable game. E.g. Imagine Tetris where you hit the Left Arrow to make a piece move left, and instead it just moves it in a random direction 90% of the time. That is what the Stronghold mini-game feels like right now.
Please fix the Prestige / Security not affecting the taxes collected, nor the percentage of taxes stolen.
Please give the Steward the ability to be an advisor to advise what upgrade to build next, or what the player should do given their character level (e.g. avoid working on any other quests until they finish Level x in Endless Paths).