One of the things I enjoy about the Key to Time season is the different approach taken to the importance of the segments in each story. In The Ribos Operation, we know exactly what the segment is disguised as, but the key is in how The Doctor is going to obtain it while safely extricating himself from the scenario at the end. We only find out about the true nature of the segments in both The Pirate Planet and The Stones of Blood throughout the course of those respective stories (and, in Stones, at the very end of Episode Four).
Romana finds the fourth segment of the key less than eight minutes into The Androids of Tara, and now the main thrust of the story seems to focus on how she and The Doctor are going to get it back. Romana discovers the segment only seconds before we finally meet the other characters in the story, leaving The Doctor, Romana, and a TARDIS-bound K-9 to set up the story by themselves. This used to be the approach taken in the early William Hartnell days, and it seems to be used with increasing frequency in the Graham Williams era.
Tara is an interesting place, a fascinating mix of old and new. It has the atmosphere and setting of medieval Europe (while Reynart's guards seem to be dressed in costumes more commonly seen in the Middle East), while every monarch seems to have at least one peasant who is highly skilled in the art of creating perfect android replicas. It's a delicious concept, and one of the more unique cultures seen in Doctor Who. There have been stories told in a futuristic setting, especially those that take place on an alien planet, but it's rare to see such a bold clash of styles, and even rarer to see the result as delightfully successful.
I had to laugh a bit at the last scene when Reynart, Zadek, Farrah, and The Doctor share a celebratory swig of wine (before passing out, of course). Farrah is ordered by Prince Reynart to bring the wine and goblets, which he does, and then is honoured when his Prince invites him to join in the celebration. But look how many goblets Farrah brings out - four. Seems like someone was under the assumption that he was going to be asked, regardless...