I myself loved this story back in the day -- got a friend of mine to tape it off TVOntario so we could watch and re-watch the swordfight (among other things), and finally bought the novelization and read it multiple times. I even, as I confessed to uktechgirl on davison_era this morning, tried to use the novelization as source material for a tenth-grade history essay on King John, and if there is any more proof of geek love than that, I cannot imagine.
Twenty-two years later, I am able to look at this story with a somewhat more critical eye, and see that the plot is thin at best and that the execution suffers from a number of obvious flaws. Nevertheless, it really is a little bit awesome.
Also, there's Tegan's paint-sploshy tunic dress, which only the 80's could love, but her earrings are really smashing; the worst fake playing of an instrument I have ever seen (as the actor playing the King waggles his fingers randomly above the lute strings); and Lady Isabella's polyester veil with glitter pastede on yey.
Furthermore, there is a jousting scene which is actually quite good, considering the budgetary restrictions; Peter Davison turns in a performance that ranges from fine to excellent, even if he can barely see past his shaggy hair; there are some genuinely clever plot twists and a couple of classic lines, particularly the Doctor's dry response to the warning that his duelling opponent Gilles Estram is said to be the best swordsman in France (which predates a similar and more famous exchange in ST:TNG's "Q-Who", in much the same way as the Cybermen predated the Borg).
As uktechgirl remarked, the Master's motivation in this episode is a little lacking... though he makes a game effort at explaining it, but as the Doctor says, undermining the foundations of parliamentary democracy in England is kind of small potatoes for him (a classic example of the old "You can't fire me, I quit!" canard used by scriptwriters aware that their plot has holes you could drive a Sycorax starcruiser through).
Ah, what else can I say? It's cheesy and I love it, and I had great fun watching it again. And now I'm off to make some screencaps, because there will and must be icons.