With the help of their Aarakocra allies the group ascended the pillar to the base of the tower, one level below the main entrance with the drawbridge. Here they found 12 large stable doors, with signs that this was were the flying mounts were kept. Listening to the doors they found an unoccupied stable and entered there. Another door led to the central staircase of the tower, but one of the Feathergale Knights was in that room. The group managed to jump him with surprise, and kill him before he could raise the alarm.
Going up the staircase they found that at each level there was a central room with doors to the other rooms of the level. They decided to skip all those doors, and just continue to go up. That worked well, because I played the tower as written, with there not being any guards on the staircase. So the group arrived just below the top level, seeing open sky above them. Landry decided to sneak a peek to see who or what was on the top landing. However he miserably failed his stealth check, and was seen by Glasstaff, who fired a magic missile at him. That started a fight, with the enemy consisting of the mage, two Feathergale Knights on giant vultures, and Thurl Merosska.
The group concentrated on Glasstaff, killing him before he got another spell off (I should have let him cast Shield as a reaction on the first attack, but forgot about that). Then Theren wanted to call a truce, persuading Thurl that they only were here because of a previous beef with Glasstaff, which he assumed to be the boss of the tower. However it turned out that Thurl was the boss, and Glasstaff his invited guest, so the group reconsidered the truce idea and the fight continued.
That fight turned out to be tough. In fact the book says Feathergale Spire could be handled at level 3, and they had a hard time at level 4. I guess the book assumes that the players don’t actually fight the air cult, because there is an option of joining them on a hunt instead and then sending them off against the rival earth cult. As an alternative the book describes the group being captured and thrown off the tower, but then being rescued by the Aarakocra. As I had played up the fight against the evil elemental cults and the information the players got on them, as opposed to the “players are in the dark about what is going on while searching for a missing delegation” story of the book, and as I had added Glasstaff to the tower, the peaceful option was a lot less likely.
Fortunately it all ended well: The group defeated Thurl and his knights on the pinnacle. They got the mage’s glass staff of defense [DM’s note: I changed that one to be able to cast Shield as a reaction, not only as an action, which would have made the spell rather useless.], they got one of the four needed keys to the magic portal from Thurl, and they used the Wing Wear wondrous item they had found in the stables to paraglide down from the top of the tower before further inhabitants of the tower stormed up the stairs. So apart from them not getting the xp for having killed everybody (which would have been very difficult) they achieved a near perfect outcome.
The group returned via the Sighing Valley to Red Larch and rested over night. Then they discussed where to go next. From Popée’s Zhentarim contact, the town baker, they got the information that the warrior in stone armor they had found in one of the four graves was presumably an earth cultist. From the dwarven book from the previous adventure they had a rough idea where the four Haunted Keeps were, and the baker told them about the Sacred Stone Monastery. They also learned about the existence of Rivergard Keep, and the information they had from Landry’s Order of the Gauntlet contact suggested that this might be where the water cultists were hiding out. That seemed to be a good next target, but as it was getting late we ended the session there.