Well, we had our first character death in eight levels of play with the Den of the Destroyer adventure.
The group fought their way through from the steps of the fortress and past the portal rooms in one go. Then after a short rest, they went north into the marshalling point. After the fight was almost over, one player - the groups Paladin - carried on straight into the final ritual room whilst the group was still finishing the current encounter, triggering the next encounter. He was hoping that the door led to a passage around to the arrow slits that were outside the marshalling point room, but was quite surprised to see the large chamber.
What followed was a tough battle on both sides. Though the group has only four heroes, they contain a hard hitting bow ranger and a wizard, both with excellent range. I can't always use the tactics provided in the encounters for the monsters, because if the monsters hang back they usually get shot from outside their attack range with magic missiles and twin strikes.
Maybe this is a common problem with many groups, but often there is little incentive to actually enter a room if you can pepper the creatures from behind a door/chokepoint. Needless to say I had to close in the Crocottas and Gnolls to attack (the powers they had were 10 square range), and though the players took a large amount of damage they fought back the gnolls until only Fangren remained.
At this point I tried to play Fangren as smartly as possible and he withdrew back to the portal, covered by the dais from range. The paladin and warlord chased in to attack while the wizard and ranger flanked either side. The paladin at this point was very badly injured, and got caught in the 'Claw of Yeenoghu' attack from the portal.
Once grabbed he took continuous damage from the claw until he escaped - only he got knocked unconscious before he even got a chance on an escape roll. I was a little unsure how to proceed now, but knowing that the Paladin had fire/necrotic resistant armour and could survive a fair amount of time, I ruled that the claw remained grabbing him. He'd continue to take 10 damage per round from the claw - reduced to 5 per round by his resistance, but it was a race against time.
This wizard at this point starting using his arcana to try and close the portal, while the warlord and ranger continued to fight Fangren. With everyone out of healing powers, the paladin survived for six rounds until a third failed death save finally took him. I described a final scene as the claw crushed the paladin in dark flame and pulled him into the portal. The wizard and ranger closed the portal one round later, sealing it.
Fangren only survived a few more rounds after that, with both the warlord and ranger bloodied with no remaining healing surges... a miraculous survival with Fangren missing his last few attacks.
The players were a little surprised to find this was the 'final' room, expecting it to be downstairs for some reason 
Well done on them (a party of four) beating the fortress without any extended rests and only two short ones. I don't think I'll throw Gilgathorn against them after that or it'll be a 'total party wipe' for sure!
They'll certainly be leaving to rest now though, so I need to work out how to encourage them to look further and find the prisoners, and how to bring a new player character into the story.
There's only one ranged striker in my group so I don't have the problem you have in combat with the party hanging back in doorways. I guess the downside to this tactic means in such tight quarters they will likely be bunched up and prime targets for AoE powers, moreover enemy artillery could make full use of cover/concealment in the room.
Sounds like the battle with Fangren was pretty climactic, just a pity the Paladin couldn't hang on for one more round. Perhaps if the party has not visited the cells yet they might find an adventurer being held there with the other captives which may solve your problem of a new PC for the deceased Paladin.
My group have had a couple of sessions with this module and so far the combats have not played as well as I thought they would. In the Mind Trap rooms the party hung back at the entrance, which made it difficult to push/slide them through the portals and they and the gnolls ended up going toe to toe and exploring the portals after the fight had ended.
What were your overall thoughts of this module and the adventure path so far?
As for the hold up with cover and use ranged, that does depend on the strengths of the group. Makes sense for them to use ranged when that is their best combat option. My group doesn't use a lot of ranged but instead has extreme mobility (the 8th level halfling rogue can clear AC 30 during movement against AoO and so often rushes in) going for them.
One thing that could be done is to make the corridor leading to the final room rather long, so it is noticeably a different area. Also, I started a discussion on the Scales of War DM group forum about the damage expressions of some of the monsters, especially when combined with situational modifiers (Combat Advantage for instance) pushing the damage to being too high. I'm not entirely sure if it is or not, but would certainly be interested in what other people have to say.
When the PC's picked up the sword in Brindol I made it a +3 Vorpal sword if I remember correctly.(the swordmage had fun with it for sure) Just before the PC's went to the portal chamber a huge energy surge from the sword gave all the PC's an extended rest. I ramped up the difficulty of the last battle a bit so it would still be hard though.
-Blackivarr
After they beat Gilgathorn, they were given a well-deserved extended rest. The final battle, unfortunately, became a bit too easy, as the party had all of its dailies available, and some poor dice rolls on my side ended up making the fight slightly easier than it should have been. The Claw of Yeenoghu missed 4 out of the 5 times it attacked.
The party took their extended rest in the marshalling room (interrupted by a small patrol) and went back to Gilgathorn who opened the door to the ritual room and started the combat. He ended up escaping and the party beat Fangren but was pretty tapped so took a second extended rest.
During that time the rest of the castle inhabitants (from the cells and training room) gathered to wait for them in the marshalling room.
They have since claimed the castle for their home base but still avoid the "keep out" room.
Incidentally, they failed the skill challenge for closing the portal and were subject to nightmares. I decided they were implanted with Slaadi and they had a nasty encounter when they went to do the cleansing ritual.