Warning: There are SPOILERS here! I'll try to keep them subtle or cryptic. But don't blame me if they reveal too much!
Better late than never, I suppose. I only recently acquired a copy of Tower of Blood, and finished reading it the other night.
I agree that it has a different overall feel from Cairn, but - personally - I like the variety. Whereas Cairn can be dropped into a campaign virtually as-is (especially if you prune out Warrens) and still provide hours of fun, it doesn't implicitly get the DM's creative juices flowing to the extent that you have to think about how the actions of the PCs therein affect the overall direction of the campaign. Or rather, the PCs can complete their goals within the Cairn and move on without worrying about any loose ends that may have unraveled. (I'm ignoring the Warrens on that point.)
By contrast, Tower of Blood forces the DM to think about a variety of subjects before or during play: What are the details of the area behind the barrier (because the PCs could easily end up going past the barrier)? What are the details for the area (two areas?) behind the portal? What are the details for the "guy behind the guy?" What events will happen if the PCs don't stop the latent machinations of the occupants of the area? Should the ramifications matter? What if the players don't want to care for that sort of world plot? Should I wait to run this module until after any related PPP modules are published?
As a result, I think Tower of Blood implicitly offers higher potential for richness in the game. Certainly Cairn can be expanded to have that richness, but you have to go out of your way to remind yourself to add it, when Tower of Blood forces you to consider the richness. And the deeper motivations of the NPCs in Tower provide a fantastic foundation for understanding how the environment may change between PC forays into its depths: What do monster survivors really care about, and how will they organize themselves to best ensure their priorities are carried out? (And yes, even the overt villain has two potentially conflicting priorities! Women, sheesh!) Tower gives plenty of foundation from which to answer such questions.
That's not to say that Tower of Blood forces itself to have a lasting impact on the campaign. Which is to say that Cairn's standalone feel (especially minus the Warrens part) is beneficial for some campaigns or parts thereof, such as when you need a standalone adventure that injects XP into the PCs prior to a plot-arc-driven, higher-level adventure. Tower can certainly be used as a similar device if you sever a few plot connections or change the superficial details of NPC motivations.
It's relatively trivial to dissociate the overt villain from the shadow world, and put a more mundane (though possibly less imaginative) goal behind the barrier. Perhaps even something associated with the villain's imprisoned love. And in doing so, you still have the makings of several evenings of exciting adventure!
Personally, I prefer to DM games grounded on the prime material plane, so I'd be likely to disconnect the situation at the Tower of Blood from the plane of shadow and substitute something else dependent on the needs of the campaign at the time the players began it. I'd probably also remove the entrance to the Spider Queen's area, and instead use her and her residence at some other point in the campaign for some other purpose. As with the flexible motives of the denizens of the Tower, the Spider Queen's goals and environment are similarly flexible.
I applaud Rob & Lance for all of their hard work on Tower (and Cairn!) for their work will allow me to be lazy and still have great fun! Very well worth the money!
Originally posted on the Pied Piper Publishing forums in 2008: