"The route from Burgos to Madrid presents few objects of interest."

He therefore gives us some idea of what it's like staying at a Spanish inn - in this case a rather unusual, rather modern one, which seems to have been built purely for the benefit of Diligence travellers.

"The Diligence halts for the night at the Venta de Juanilla, a solitary edifice situated at the foot of the last or highest etage of the Somo Sierra, in order to leave the principal ascent for the cool of early dawn.  The building is seen from a considerable distance, and looks large; but is found, on nearer approach, to be a straggling edifice of one story only."

(He's wrong about it being a 'modern' inn. As far as I can make out the Venta de Juanilla - which is still around, still trading - has been there since the 10th century. But it's had quite an upgrade for the 21st century - as indeed it must have had for the 19th, if Nathaniel thinks it's 'modern'.)

"It is a modern inn, and differs in some essential points from the ancient Spanish posada, - perfect specimens of which are met with at Briviesca and Burgos.  In these the vestibule is at the same time a cow-shed, sheepfold, stable, pigsty, - in fact, a spacious Noah's Ark, in which are found specimens of all living animals, that is, of all sizes, down to the most minute; but for the purification of which it would be requisite that the entire flood should pass within, instead of on its outside."

Hang on there.  Is he saying the traditional, typical Spanish posada of his day has its ground floor entirely devoted to stabling farm animals? Presumably the much-valued property of droving/farming hotel guests, and placed there for greater overnight security?

Immediately the problems associated with this arrangement become clear. But he spells them out for us anyway.

"The original ark moreover, possessed the advantage of windows, the absence of which causes no small embarrassment to those who have to thread so promiscuous a congregation, in order to reach the staircase; once at the summit of which, it must be allowed, one meets with cleanliness, and a certain degree of comfort."

By Wouwerman, a Dutch painting, not Spanish; gives the idea, though.

So, when entering the typical posada you're coming in from brilliant sunlight into what is effectively an unlit pigpen, maybe with a few sheep or goats wandering about in a state of uncertain temper.  You pick your way half-blind across this farmyard floor - lord knows what you did with your skirts if you were a lady - until you make it, hopefully without skidding in any doo-doo - to the staircase.  At the top of which would be your accommodation.  Nathaniel's Spain is not designed for tourists.