Disclaimer: I am not J. K. Rowling. I do not own Harry Potter.
Note: This piece is by way of an experimental attempt to nail down some details of geography and history of Hogwarts for the 'Saint Potter' alternate universe. Some details will almost certainly differ from canon. This piece is rated 'T'.
Further Note: The piece is an account dictated in the late twentieth century by an 'in universe' character, who has their own views and agenda, and may be biased, guessing, or otherwise misleading on some points.
Extract from 'Aquinas' Guide to Hogwarts'
The rising number of apprentices brought about demands other than for the time and wisdom of the four founders, of course. All those (generally) young men and women required somewhere to lay their pallets, and to store changes of clothes and other personal possessions. And whilst one apprentice (Adam of York) had been relatively easy to tuck away somewhere and largely forget about, dozens were soon getting everywhere, getting underfoot, and generally creating an ill-tempered atmosphere with arguments about space. The situation was further compounded by the fact that romantic fixations or visiting relatives expected to have somewhere to be able to lay their heads – and this was in an era before Hogsmeade had even been thought of, and in the midst of lands some of whose residents were very much resentful of the castle (and anything to do with it) which had arrived in their midst. Visitors expected to be able to stay in the castle itself.
Godric ultimately surrendered to the needs of the situation and sent for the giants again. A further tower was added to the castle, this one abutting the southernmost corner of the hall (with the length of the hall between it and the founder's own apartments in the keep). Here, in this new tower, all the apprentices and their hangers-on were housed. This latest addition to the castle would with the passage of time become 'Gryffindor Tower', but at this point in the castle's history there was as yet no formal school or 'house' system, and the apprentices – although numbering in the dozens – were not so many that they (and their occasional visitors) could not all be comfortably accommodated together in one place.
Meanwhile, whilst the giants were handy, Rowena had Godric and Salazar and their crew create a walled garden, for the convenience of sheltering plants which Rowena considered it necessary to cultivate for her own research from some of the extremities of the Scottish climate. Besides the erection of the protective walls, the creation of this garden involved the bringing in of a good deal of high-quality loam; this latter must have been a considerable labour, given the awkwardness of transporting large volumes of highly friable material from a source likely much more remote than the Rannoch granite quarries. That the use of magic in said endeavour could have rendered the soil worthless for years for the growth of certain plants sensitive to lingering traces of magic was one further challenge with which the founders would have had to wrestle.
Work on tower and garden projects complete, Godric and Salazar were permitted to dismiss their giants again. No doubt there was some muttering on the part of the men as to why the ladies hadn't at least mentioned the garden the previous time that they'd had the giants in, even if the need for such extensive additional guest accommodation on a permanent basis was not something that could have been reasonably foreseen.
Author Notes: (updated 14th March, 2016)
'Pallet' is used in this chapter in the sense of a rough bed.
A walled garden isn't like to keep much of the snow of a Scottish winter out, but it should provide an enclosed space where the effects of wind (including of 'wind chill') are all but negated.
For the purposes of this universe, I'm assuming that active magic or 'after traces' of magic in an environment can impact the properties, as potion ingredients or otherwise, of any plants grown there.
Nearly there as far as the earliest days of a formal school are concerned.
Update, 14th March, 2016: As one early reviewer has reminded me, a walled garden also offers protection to the enclosed plants from at least the larger representative member species of any local wildlife (whether fantastic, such as unicorns, or more mundane, such as deer) which might otherwise attempt to browse upon them.