The thing that's striking about 'The Roof of the World' is the way that it delights in playing games with its audience's heads. It's a trick that perhaps slides past us modern viewers if we're not careful; in retrospect, this is the first "historical" story and a classic example of a common sub-genre of this era of Who. But that's in retrospect. At the time, the series had never done a proper "historical" story--we'd had a bizarre drama playing out among prehistoric humanity and a science-fiction epic, but many in the audience had no idea what to expect when the travelers next stepped out of the TARDIS. (The Radio Times had published an article on the upcoming story, but it was a far cry from the modern spoileriffic Internet.)
And Lucarotti's screenplay plays with that for most of 'The Roof of the World'. The first thing we see is a footprint larger than a human being could make...which is promptly dismissed as a half-melted bootprint. Barbara sees strange, furry monsters...which turn out to be normal people in furs. Tegana, warlord of the Mongol Noghai, tells of shapeshifting evil spirits...but Marco Polo dismisses it as superstition, and it's not brought up again. Marco, when confronted with a "flying caravan", speaks of it as part and parcel of the same magic that allows Buddhist monks to levitate cups of wine to the mouth of Kublai Khan...but this is presented as a mere unusual background detail to what is otherwise settling into an adventure with a well-known historical figure in a well-documented historical period. Everything seems to be settling in...
...until Marco announces that he wants the strange, miraculous flying caravan for Kublai Khan, and we realize we've been looking in the wrong place for aliens and strange science-fiction weirdness. It's not the situation that the Doctor and his companions have stepped into; it's them. They are the strange aliens that have stepped into Marco Polo's story, and far from being the first example of a classic historical, this is the exact opposite. This is a science-fiction story, and the TARDIS--that wonderful, impossible piece of futuristic technology--is the key to it all.
But it's also, fundamentally, a personal conflict in the way that the early Hartnell stories managed so effortlessly. With the tension more or less resolved among the TARDIS crew, Polo takes the place of Ian and Barbara as a sympathetic character who nonetheless has goals that conflict with the other protagonists. The Doctor has resolved himself to helping Ian and Barbara get back to England, but Marco is willing to do whatever it takes to reach his own home. And he's utterly incapable of understanding the bitter irony in his statement that Ian and Barbara can get back, but "it would take some time".
With all that, it's almost overkill to find out that Tegana's planning to kill everyone. That may be the cliffhanger, but the climax is the Doctor giggling maniacally at the thought of Buddhist monks piloting the TARDIS. His world is so alien to Polo's, literally and figuratively, that he's reduced to tears of helpless laughter at the thought of reconciling them. A brilliant end to a brilliant episode.