Well, they are all just exactly where I left them. It's just that "Wither Felltower" sounds better than "What the heck is up with Felltower?" or "Where is Felltower at these days?"
While I'm at it, let's look at Stericksburg and the Cold Fens, too.
Felltower is still physically where it is - under the mountain, sitting just a half-day's hard hike from Stericksburg.
It's essentially static. That means the PCs aren't going to show up and find the orcs have established a burgeoning metropolis on the top of the mountain. Although that would be funny.
But it also means they aren't going to show up and find the orcs are gone, leaving the depths of the dungeon open for plunder.
on the other hand, it will have been re-stocked to a larger extent than usual. Rooms emptied of monsters may have become occupied again. Tunnels blocked by junk may have been cleared. Doors may have been knocked down. Loot looted. Etc. I've started doing a slow pass across the dungeon and making the changes that keep it "alive."
It will be a similar challenge - and similar scope and depth and level of challenge - but not the same challenge exactly.
Short version: Don't assume the entrances and exits are all exactly as left, and that all encounters are frozen in time from the moment Mungo's hammer fell that last time.
The last actual PC visit to Stericksburg was in March, 2015. It'll almost certainly be one year from the previous visit from when we next stage a game out of Stericksburg.
This does mean a little bit of degrading of the magic item market, special item market, etc. - PC spending matters, and spending hasn't happened recently. I only track PC spending in a rough fashion, but I treat it as representative of overall economic activity. The PCs aren't buying stuff? Other people probably aren't, either. The PCs and their cash are the liquid assets in the economy.
Unfair, because I'm the one that pushed them away from Stericksburg? Sure, maybe. Unfair like having bad weather in a wilderness game is unfair, in my opinion. No upside but it's reasonable, realistic, and adds to verisimilitude.
Partly this is just to stop the inevitable "spend everything I earned in the Lost City all at once on a single big-ticket power-enhancing item and then hope I find enough money for upkeep!" approach. I do get a little weary of PCs looting a lot of money and then being broke right away - not because they frittered it away in Conan-like fashion but because they bought a magic sword or a bracelet with Missile Shield on it for 98.5% of their ready cash. Cash isn't like XP are, where unspent it's useless. Having a reserve so you each mission isn't "plunder or die!" is a good idea.
That's less self-contradictory than it sounds - since Stericksburg is only rolling back on the top end (magic item sales, powerful material sales, availability of top-end anything) that leaves the players a bit less of an outlet to spend everything the second they arrive. But it also means the usual consumables and items folks depend on are there. Torchbearers, healing potions, etc. - no problem. Off-the-shelf items of power? A much harder roll. Probably a waiting list ("That'll take 100 days to enchant - I'll start in 3d weeks!") instead of actual less chance for custom items. I should have put a waiting list there in the first place.
Short version: Don't expect to blow all your cash on a power upgrade, but do know the basics will be there in their usual abundance.
The Cold Fens are still out there, too. It's quite possible the damage done there has set back the Bad Guys a lot further than elsewhere. They'll grow and recoup and otherwise change over time but the PCs really did smash most of their ability to deal with the outside world.
On the other hand, the great evil lurking below the swamp is still there, and the Church wants it gone. Their hand-picked strike force was exterminated down a single survivor, who didn't owe them anything and who took off to greener pastures.
The dragon is out there. The two other rumored ruins besides the are they were exploring are also out there - and yes, it's possible the dragon counts as a third location or as one of those two.
The Cold Fens is a possible stopping point on the way back to Felltower if the PCs really feel like they need another side trip and want a very hard one. I expect they'll pass it up. The longer it is left, the more powerful it will get. But equally, it's a powerful and dangerous place now, and it might be too much at the moment. We'll see how they choose to deal with that after the timer runs out on The Lost City.