Did you complete the fortress?
Twice so far, once each on a copied / template alt (due to later bugs blocking progress, I might take a copied alt who hasn't completed the Planar Vortex quest through before the next patch too)
My copied alt was CP1620 with a pretty hardy stamblade 'brawler' build, and decent but not raid-level gear, and I pretty much copied her set-up to my template CP3600 alt and took some time to sort all her points etc before getting started, since I knew what I was in for.

How many players were present?
On both I solo'd the Infernium and a lot of the 'running' content, occasionally I'd cross paths with someone while firing at a gate, or run through some groups of mobs in tandem with a stranger for a while, but I only twice had more than a single other player along while travelling to reach a boss.
I was lucky to have a few mutuals arrive and group up at the Harvester on my first/copied run after attempting to solo it for around half an hour and being constantly interrupted by the respawning mobs. I'd taken maybe 10% off her healthbar, trying to repair the ballista for another two shots at the wards left me able to be interrupted by the mobs and she'd reset to being fully warded, so progress was difficult.
Similar stories with the Colossus - without numbers I was stuck grinding with minimal progress.
I think soloing the whole thing is currently almost impossible, except for those few crazy enough to regularly solo their way through vhm DLC Dungeons, (but just 4 competent builds / people who could see what was needed were able to get it done, 6-8 would be better, and 12+ would make it pretty easy).

Which was the hardest encounters? Easiest?
This is on the assumption of there being a dozen players, or a smaller group able to co-ordinate well:
The Harvester would be the easiest, except that the colours for the wards are so faint that I think a lot of people can't tell which they're aiming at, and some vision-impaired players will not be able to tell the difference between the blue/orange at all:


The Colossus can be confusing for people who've run in along a pre-cleared road and arrive on the platform then stay there, not realising there are things to clear below. I'd sometimes spot one standing around doing nothing when I took shortcuts over the top to get to where the next target was while avoiding the plasm.

And the final boss is definitely the hardest with the various mechanics, and may be a problem with larger numbers because of people having to wait for the orbs to respawn to refresh their shield if another player beats them to it, these are going to become camping points slowing the pace unless they can be spawned in faster, or otherwise changed to account for a crowd.

Was it fun?
When it wasn't deserted and tedious, yes, very much so.
I love the scale of it, and I think this is the one time I'm actually looking forward to the madness and crowds of the first weeks post-Update on Live, because it's going to be utter chaos in the best way.
Being able to backtrack and help the other NPCs after they spilt up was a relief, too, I was worried the initial choice would lock me into only half the content.

Did the time to complete feel right?
Again, depends on numbers, they made a huge difference to the boss fights.
Is it possible to scale the mobs' respawn rate/numbers based on the number of players present in the instance to make the experience more consitent regardless of how many might be there?

How long did it take?
As mentioned above, soloing the Harvester I achieved maybe 10% over half an hour, but once there were enough players that a couple could stay on the ballistae full-time while others dealt with the mobs or making repairs, she melted pretty quickly.
Second run (on template) was 20-ish minutes with 4 people present for the chest once a few friendly strangers turned up, and we moved on and got the Colossus in just under 10. If not for wiping with barely 5% to go on the Titan it would have been about 20 minutes, still the same 4 people as far as I could tell. I had a disconnection during round 2, and by that point more players were arriving so round 3 was already half over by the time I reconnected and sprinted through to rejoin.

Any additional feedback?
Overall it's pretty self explanatory - I like that we're not given as much hand-holding in dialogue screens and are left to follow shouted directions, it feels much more realistic.
Less repetition from the NPCs giving us instructions, would be good though, please... "Yes, Walks, I know I need to use the Coldfire Ballista against the blue wards, but I'm drowning in mobs right now, stop shouting and help me..." is probably the most polite thing I can repeat here from attempting to solo the Harvester, but it got a little repetitive elsewhere, too, and was sometimes contradictory to what the actual task was as things progressed.

Before we leave the Rampart Camp to head to the breach/portal to the Writhing Fortress, if Darien is spoken to he'll have this line:

The implication seems that he's coming with us, but he stays behind.
Is he supposed to be a follower for this? (please, please, pretty please?)
Or is the line supposed to be related to something else? It happened on both copied and template alts.

Once at the Wall, the directions were inconsistent and a bit confusing:

Skordo speaks to the soldiers guarding the golden Breach to Eastern Solstice, before he runs though that rather than entering the blue Portal to the Writhing Fortress.
The map marker also indicates to go through the breach instead of the portal:

Did you complete the fortress?

Yes. I completed it on my copied main character who had progressed through the Solstice questline up to the objective to clear the Fortress.

How many players were present?

It's hard to say since everything was so chaotic and people were spread out throughout the instance. I'd guesstimate around a dozen by the time we all got to the last boss. Probably 4-6 on the harvester boss, which was the first one I did, and then more joined in as we made our way toward the last boss.

Which was the hardest encounters? Easiest?

They were all probably harder than they needed to be, given that I spent much of my time not really knowing what I was supposed to be doing, so we were doing a lot of flailing around without making progress. Also I missed the boss on the east side, so I don't know what that encounter is like.

I'd say the harvester wasn't bad once enough people understood the ballista mechanic to deal with the wards, so maybe that was the easiest? And the titan was probably the hardest because there was a lot going on and it was hard to even target the big boy.

Also they would all suck without a lot of people, so I second the calls for dynamic scaling based on how many players are there.

Was it fun?

Ehhhh...

It'll probably be more fun on Live with more people and more who know what they're doing. I enjoyed the epic feeling of a big group assaulting a wide-scaling fortress and making progress on multiple fronts at once, so the concept is good. But in practice, especially without a big group, it feels like more of a slog than an exciting dungeon rush.

Particularly for my first time going through it, I'd say it was more confusing and overwhelming than fun. Other than disembodied NPCs shouting out vague instructions, and the one Stirk Fellowship mage guy who told me about the balls I could break for a shield, I had no guidelines about what I was supposed to be doing so I had to figure out a lot of stuff on the fly (like the orange/blue ballista mechanic and the fact that there were grapple points to get around). I got the sense that a lot of other people were also confused and just fighting whatever mobs were swarming them without a clear objective.

Also after talking to a few people I have realized that I missed a bunch of stuff including NPC interactions that are supposed to happen at the beginning of the instance. When I got there, the only NPC I saw was Keshu, so I wasn't able to talk to Azah or Walks-in-Ash or Mirmal or anybody else. I presume this is because the instance was in-progress and a lot of the objectives had already been completed. So if any of those NPCs had helpful explanations, or if any of those completed objectives were designed to teach me about mechanics, I missed out on all of them. (I'm also annoyed in general about not getting the chance to talk to everyone.)

Did the time to complete feel right?

It took significantly longer than I expected, especially for a necessary step in main quest progression. It'll probably be faster on Live with more people, but I'll still need to set aside a substantial amount of time to make sure I can complete it in one sitting. (Especially since if you can't complete the final objective before you have to leave, it means you'd have to start all over again with a new instance, so all that work is wasted.) And since the time investment is completely dependent on the number of people and the amount of wheel-spinning you have to do to get anywhere, the probability of a drawn-out slog is relatively high.

How long did it take?

I spent about an hour actively participating. Some objectives had already been completed, so I'd estimate that I experienced only about 40% of the content in the instance within that timeframe.

Any additional feedback?

• The orange and blue wards need to be made waaaay more distinct. It's way too hard to tell that there's even a difference between them.
• The encounters that involve ballistae get significantly less wheel-spinny once the barrels of repair parts spawn in. I presume that's something that happens after a certain amount of time has passed? If we have to rely on the green circles to spawn after killing mobs, then we end up spending more time grinding mobs than using the ballistae to actually progress the boss fight. So more barrels plz.
• Where is Darien during the Writhing Fortress operation? His earlier dialogue suggests that he's coming with the group, but then he's nowhere to be seen. If he's staying behind to guard the main camp or something, then this should be acknowledged either by him or by another NPC. (And if the choice to have him stay behind is related to his "condition" after recent events, then it would be good to have that established too, and to know if that was his choice or if another NPC insisted on it, and how he feels about it in the latter case.)
• Please find a way to make the grapple points way more obvious. I spent a stupid amount of time running through bathwater trying to get back up to a bridge before someone told me I could grapple up there.
• The mob spawns could stand to be toned down, especially in areas where the actual objective involves interacting with the environment (like repairing and firing ballistae). Especially for solo players or small groups, the trash mobs can completely interfere with progress.
• Having the portal entrance (the blue circle) right next to the gold tunnel to eastern Solstice is confusing, especially since the quest marker is currently marking the tunnel. Also that's where Skordo ran when he was leading the way to the Fortress. Like you're going the wrong way bro.

A couple more things to add while attempting to run my third alt through this:

For the Titan:
Please give us a barrel at the entrance before it starts. Just one, back near the landing platform, please?
Otherwise it's the usual feedback of the grappling hooks needing more range on their activation points, the shouted instructions becoming annoying (especially when it's been a few hours because it's deserted).

I can't wait to run this with proper crowds, and then never have to ever do it again unless I'm feeling nostalgic and deluded that it wasn't really as soul destroying as I remember it being, surely?