I have, in fact, been running my campaign for a while, so I'll offer some reflections for Alex S's recent inquiry. We play 5e D&D.
How many sessions have you been playing, more or less?
Give or take 70.
How long have you been running this campaign?
Since October 2015.
Have you had long breaks? If so, how did you pick it up again?
There was one or two points around the 1 year and 2 year marks where we didn't play in at least a month.
How many people are at the table when you play?
Between 3-6. Averages 4.
How many characters are in the party when you play?
Each player controls 2 characters, but only plays one at a time. This was due to some shenanigans I'll explain later.
How many players have you had in total over that time period, not counting guest appearances?
11. Of the initial 4 who I began with, 1 remains in full and 1 remains part time.
About two years in, due to lack of activity I decided to revamp the campaign and begin a second play group made of entirely new people acting simultaneously within the campaign world. Eventually, as numbers dwindled, I combined the two groups again into one.
Have you had guest appearances? How did it go? Did you gain regular players that way?
Sometimes we'd have one of my siblings make a guest appearance, or had someone's husband show up. It went pretty fine. Usually they were quite impressed with how I ran things.
What have the character levels been over time?
Levels 1-13. Rapidly progressed over the first two years until level 11. Then, when the second group was formed, I brought peoples' levels down to be closer to newcomers. They've steadily crept up and equalized until they're now at 13.
What classes did the players pick? Did you add new classes over time?
We've had everything at one point or another except a Sorcerer. We added the Artificer when it came out recently.
Tell me about some adventures you ran over that time that I might enjoy hearing about?
I think my best one was an assassination plot against the party. I used one of Arnold K's d6 Assassins, Wizened Grudlow specifically. Had him announce his arrival 24 Hours ahead of time, like all good assassins do. In a metagame effect, I wrote down all of Grudlow's resources and tactics on small sheets of paper, folded them up, then put them in the middle of the table before the players even started planning their defense. I then let them prepare.
Party did some information gathering first. Found out that an Archmage they'd recently stolen from had hired the assassin. They then broke into the Bureau of Spatial and Temporal Matters, went back in time to try and alter the present, and spectacularly ensured that the assassination would occur. Grudlow, even in the past, out-foxed 'em. At least, though, they knew some more about who was coming after them.
They prepped a holdout at the Black Magic Fraternity's headquarters. Clocks placed every square meter to track Grudlow as he approached. Exploding suicidal zombies. Black Magic Brothers armed to the teeth. A Thousand Curses to be flung upon he who crossed the threshold.
Grudlow approaches. Time slows down. I'm playing that assassin theme from Kill Bill on my laptop, but distorted and reversed. I unveil Grudlow's chef strategy, placed there at the start of the session: he's going to burn the place down from the outside and watch everyone slow roast. He throws two torches in through the front door.
Suicide zombies out. Next preemptive strategy revealed: sniper on the roof. Nails the zombies, they explode on contact. Place is on fire. Party leaves through the back alley, having seen what happened to the zombies. They had Haste potions before, so they use them to escape the flames. In the back alley, they get ambushed by a shapeshifter and a trash wizard. Deal with them easy enough, despite their Haste as well.
After that, they approach the main alley where Grudlow is waiting. Monk takes a peek, avoiding a shot from the sniper. Fighter takes the next shot on a shield while the monk uses their Adjacency Glove (TM) to become adjacent to the sniper, then monk-kicks him off the roof.
Grudlow's got two lackeys with him. I reveal the next bit of paper: a hostage. He took an important employee of the party, and now he had a knife at her throat. Monk wins initiative, thinks he can save the hostage with some quick moves. As this happens, the last bit of paper gets revealed: the hostage is a disguised assassin. The monk goes from full health to dead in a single turn, shanked by a poison blade and cut to pieces by Grudlow.
Then begins the chase. During which, the lackeys take some hits and leave before going down. Grudlow pursues to the bitter end. With his constant Slow aura he's able to keep up with the party no matter what. Three turns of the luckiest Hold Person saves in favor of the party keep Grudlow frozen while the party beats him down. Barely. Of the 4 PCs that started: one dead, one unconscious, one mutilated, and one standing.
Have the rule changes over that time? Do you maintain a house-rules document?
I do maintain a house-rules document, or rather, a couple. We began RAW, then an injury table was added, then Saving Throws for certain abilities and spells were shifted around. Then I threw out the WotC Monster Manual entirely and stuck with my homebrew.
Has the setting changed over time?
It has. Things shift around and change whenever I think of better ideas for them, or the old ideas run out of juice. A lot of my old material has been replaced. A recent example was that I entirely replaced my old Wizard City with the entirely new Wizard City that I'm writing on my blog right now, but having the city zip up in a bubble and undergo a 50 year timeskip.
How much in-game distance did the party cover, how big is the area they have visited?
They have access to an airship, but the world of Tidelock is very compressed. They've probably covered an area the size of India.
Have you used proprietary setting books? Like, could you publish your campaign or would you be in trouble if you did?
The campaign world is like... 5 campaigns worth of materials. Nearly all of them have some proprietary adventurers thrown in to fill up the gaps, but nothing that couldn't easily be excised. I'd need to change the names of my Archmages - they're a bunch of Greyhawk character names.