Durlag's Tower is the lesser prequel to the far superior adventure that follows, "Durlag's Tomb." The encounters are a little forced, the treasure/loot is frustratingly overly complicated, and the puzzle at the end doesn't quite engage effectively. SPOILER BELOW, OF COURSE
Part 1. In my opinion, the tea party was the best part of the adventure. It was good to see SEER and Hsing again. Players really need to ask a lot of questions to get the information out. There are about 10 bullet points worth of information that could be important, not only for "Tower" but also the "Tomb" adventure. Anxious adventures may skip some important details at their own peril. Oddly enough, it also connects with aspects of the season 5 epic "Ark of the Mountains."
Part 2. Much time was spent wandering the grounds figuring out what to do. (Expected duration: 180 minutes) Some of the monsters in the encounter can feel a little out of place. Make sure that the background information on page 4 ("...legend also has it that many powerful demons, devils, and undead now inhabit that place") is communicated effectively. Otherwise, it feels like the randomest of encounters.
Side note: Mechanics behind the "hizagkuur" stone is a tad frustrating plot device to keep people from teleporting. (Similar efforts were made with Eye of Xxiphu stories. Do we just not like teleportation spells in tier 3 and 4 anymore?) If a wizard doesn't understand the effects, unknowingly tries Misty Step and takes 10d10 force damage that could instantly kill the player. DM's be warned.
F. Wooded Copse was the most interesting encounter of this random section. Poor, poor Bitter-Root.
Part 3. The shorn, the hunter, the delver, the crafter: "A bit of simple trial-and-error" should not be the basis of a D&D puzzle.
Schvalt was a good boss fight. I assume the "hizagkurr" is here so that a banishment spell does spoil the fun too quickly. But the demon being able to reflect spells is a nice twist to make things a little more challenging.
If the characters don't find the invisible lockbox (why would they?), they aren't getting much treasure from this adventure. Also, on page 14, the malachite is worth 250 gp, but in the rewards section it says it's worth 1,500 gp. (Quality control!) And if the party chooses not to sell the jade dagger they find, SEER will give a character (one?) a 2nd level spell scroll. Seems like an unnecessary trade off to overly complicate every aspect of the rewards section.
The Traitor to Your Faction "reward" (?) also overly complicates matters. Can you imagine two adventures from now telling a different AL DM that you can't take the renown because of reasons back at Durlag's Tower?
Vergadain's Firespirits: While I like the idea of using downtime to craft "firespirits," when you start involving Fai Chen, you know it's getting a tad convulted for it to be practical for most players.
Quasit's Essence: Destroying the Staff of Thunder and Lightning in order to get use of a quasit familiar for a wizard with a quasit familiar? Also, convoluted.