He truly hated the stairs.
He was not in a hurry; not on any urgent Hogwarts business; not rushing to a student's aid. He was simply walking from the headmaster's office to his own, reminding himself to check the progress of some of his otherwise-unattended brewing and reviewing his procedures when it happened: a step on the grand staircase tripped him. It had been nearly two years, and (he hated to admit) he had not been on his guard — and nearly took quite a tumble for his carelessness. The stairways had a long (and somewhat vicious) memory. That much was certain.
It was a never-established fact, like so many of the elusive certainties that were Hogwarts: absolutely no one knew why the stairs moved. There were no records, no indications of the Founders' intentions. There were no enchantments that could be perceived, divined or dowsed. They were neither (as a particularly gifted headmaster of the fourteenth century, Cognitus Durer, had hypothesized) permanently transfigured kobolds, nor were they the magically-manacled, glamoured and time-locked arms of particularly stupid giants frozen in the castle bedrock, as a much later headmaster, Cryptocolens Lovegood, had surmised. But they moved nonetheless.
True, they had been known to harass invaders on the two occasions the castle had been breached during the Goblin Wars, but for more than a thousand years they had been, almost without exception, just a pranking nuisance to students and professors alike. They began their irritating dance before dawn, the day the students arrived. Come sunset of the last day of classes, they were still — and would not budge all through the summer. During winter holidays they were sometimes boisterous, sometimes quiescent — there was some vague correlation with Yuletide celebrations — but nothing unequivocal. They were, quite simply, thought Snape, a dreadful, bitchy bit of the architecture that were determined to make his life a mobile hell.
Potter, Black and Lupin had never seemed to have much trouble with them, but him they despised. In reality, itt had not been that much of a problem to stick to the lesser spirals — and avoiding the central staircases, while favoring the shadows of the many "secret" passages between floors, had (inadvertently) contributed to his reputation as one of the sinister elements. An unexpected boon. And, in any case, he'd favoured Dark Arts and Potions, both of which were taught in the dungeons. So his problems were few. He had gladly left "the stairs of Hogwarts" behind him when he graduated. (He had not failed to make more than a few rude gestures and wand waves at several flights of stairs after he'd completed his NEWTs.) When he became a professor, though, it seemed that they had renewed their persecution with a certain — vigour. The battle had persisted, if not raged, for the first handful of years of his tenure. And he was rarely victorious.
He had thought he would have them on the ropes when he learned the a-'a incantation, a particularly brutal stone-melting curse (just a notch below Fiendfyre, actually) from the Pacific islands. Sadly, his hopes were not realized.
He could never decide which had been more unnerving: the fact that his "ultimate weapon" (used during an especially trying nightly round) had been completely ineffectual, rolling off the second floor flight of stairs like so much rainwater, or that Dumbledore had confided on the following afternoon (with a look of deep, fatherly concern), "Perhaps, my boy, you shouldn't venture much further 'up' than the Great Hall for the next week or so. And —" in a more hushed, even fretful tone "— if, thereafter, you hear barely audible whispers of "pahoéhoé" that seem to come from the walls — making your way to a window or balcony might be prudent."
He truly hated the stairs.