I'm not going to say that Dennis Spooner isn't an immensely talented and influential writer who really transformed Doctor Who from the slightly staid, educational series that it started out as into the adventure series it became. It's not an argument you can seriously make. But I will go so far as to say that there are weaknesses to his approach, and "Guests of Madame Guillotine" serves to highlight them.
Because like I said about the previous episode, Spooner's approach is to take us out of Historical Revolutionary France and into Storybook Revolutionary France. We're no longer sourcing our material from the history books; we're sourcing from 'A Tale of Two Cities' and 'The Scarlet Pimpernel'. The heroes are wandering through the tropes associated with this era, rather than its lived experience. Now to a certain extent, that's a very intelligent move, and it's part of the reason that Spooner's scripts always feel so modern--characters in most stories now are aware of the tropes of the stories they're in, and consciously exploit or avoid them on a frequent basis. Stories like 'Cabin in the Woods' and 'Scream' do this all the time--it's just a feature that comes fitted as standard now, because the audience is so steeped in awareness of the structure and forms of drama that it seems unrealistic to have a character in a story do the expected thing when it doesn't benefit them. Spooner is basically inventing postmodern television about forty years early.
But the problem with this is that once you've committed to that decision, you have to embrace it. If the characters are consciously intended to be characters in a story, aware of its tropes and embracing the structures and forms of drama, then you have to actually embrace the structures and forms of a drama. Which means that if the main characters get locked up in a filthy, miserable jail cell in the middle of the Reign of Terror...they have to escape with ludicrous ease, because that's characters in adventure stories do. They just do. A prison cell is a narrative device to stitch two plot threads together in a story like this, not a serious impediment or threat to the heroes.
But this is a six-parter, and a six-parter in Doctor Who usually has at least one stretch of time-filling. The trick is to conceal it. 'Marco Polo' concealed it with local color and historical detail, but Spooner has consciously jettisoned that approach. And as yet, he hasn't quite figured out what to replace it with. So we get an episode that's pretty much a full-length stall (which isn't helped by the fact that Ian's only appearance in this episode is in a couple of pre-filmed inserts, a la Bela Lugosi in 'Plan 9 From Outer Space'. And yes, that particular comparison is deliberate. And no, it's not intended as a compliment.)
The only bit of actual plot in the entire episode is Webster's dying confession to Ian. Everything else is an exercise in killing time until the end credits run; Susan and Barbara dig a tunnel only to give up when it gets hard, and the Doctor spends the entire episode walking to Paris, with the exception of a brief and pointless sequence where he apparently beats a man to death with a shovel. These are things that should be handled by a montage, or better yet a dissolve to the next plot point, but instead they're shown in detail. The Lucarotti model could get away with that, because it was at least making a pretense at realism. You could justify spending a whole episode in a grotty jail cell, with Susan despondent and Barbara trying to cheer her up, because it's the sort of thing that really happened to people in jail cells and they really did get miserable and desperate and terrified. They even try to reference their imprisonment in 'An Unearthly Child', but the atmosphere is light-years away.
This is storybook jail, now. Storybook jails exist only as a place to leave. Which means that this episode, in the Spooner model, doesn't need to happen. As he gets used to the style of television he's creating, he'll come to realize that, but for right now, the longeurs of the plot are all too exposed. Which leaves us just as frustrated and trapped for most of this episode as the characters themselves.