The Pyralis Effect is a standard Doctor and aliens runabout. My expectations for this were pretty low, based partly on steve_mollmann's review but largely on the fact that it is by George Mann, whose fiction and non-fiction has failed to impress me. The fact that it more or less held my attention to the end has to be considered a major triumph, and (given the discussion in the extra tracks of the number of rewrites extracted from Mann by Big Finish) a triumph shared by many. Let us consider it equivalent in quality to the average Season 17 story, and leave it there.
On the other hand, I quite liked The Castle of Fear. Partly, it made me nostalgic for The Kingmaker, which is one of my favourite Big Finish audios; it's not as good, but then few Who stories are. I hate John Sessions, and luckily all the bits I thought weren't funny enough were the bits with him in, so I was happy enough to enjoy the rest. A clever plot, just about as funny as it could bear (Sessions apart), and good stuff from Davison and Sutton. John Sessions fans (the mad, deluded fools) will like this one.