Harry watched as the Hogwarts he knew formed before his eyes, adding suggestions here and there which were quickly agreed upon. There was a brief magical discussion about where to put the students, and it was decided that there should be four houses, and that there should thus be four common rooms. The common rooms were placed in a general area on the plan, but left for when the building started. The Great Hall went through a number of iterations, from round, to hexagonal, to octagonal, even resting lightly on magical pentagrams and heptagons, seven being the most magical number. Eventually, when the kitchen had been perfected, the Great Hall was planned to mimic it, not the other way round, as so many people would think.
Harry put forward the query of the names of the houses, and it was quickly agreed that they should be named after the founders, Gryffindor, Ravenclaw, Hufflepuff and Slytherin. It was also agreed that the founders themselves should build the common rooms. Everyone had chosen a portion of the castle that they wanted to be responsible for, where they wanted to plumb their own secrets. No one person would know all of Hogwarts' secrets unless they were to discover them during their time there.
The next day dawned bright and pure as the sun rose over the large tent the helpers had erected. Inside it was even larger, as was obvious from the sheer number of people leaving the tent and heading for the ruined castle. Not everyone was going to be building the castle, as it had been decided that a few people needed to improve Hogsmeade, and some were in charge of sorting out the transport to the castle.
As soon as everyone was ready, each witch or wizard began creating their respective areas, conforming loosely to the guidelines they had set up last night. Harry found he could direct what Godric wished to create and look at what else was being created at the same time. It was such a sight to behold, with huge chunks of building floating and revolving in the sky, some with people clambering over them to fine-tune their work. Harry could see Helga developing the kitchens and casting some of her culinary magic over the area. Rowena was building the library and some specialised charms rooms on the floor below. However, Salazar was nowhere to be seen, and was probably hiding behind one of the many hills around the castle. Presumably he would be sliming around in the dungeons.
Harry, however, began to form most of what was planned to be the seventh floor. He created an office just to get the hang of the magic, then started on his cunning plan. He would build a room like the cavern. You could hide in it, and it would be inaccessible to all others. If you wanted, it could give direct access to the lower floors through portraits. It could become whatever you needed it to be. It would be any size, any shape, and it wouldn't alter the plan of the rest of the castle in any way. It would be a room of pure magic. He decided to make the headmaster's office too, as it would need a good vantage point.
After this, he relaxed, letting Godric back in control and having a look around. Godric, of course, thought he was always in control, and never once considered that Harry was inside his head. Godric began to work on the Gryffindor common room and Gryffindor tower. Harry, meanwhile, peeked at the secret passageways the three Irish wizards to his left were creating. They all seemed to have a trick step in them, and would be scattered across the whole castle. A sensible witch was creating the staff room just past them, hiding secret compartments for the teachers to find. What Harry saw, he filed away as he knew this knowledge would be useful in the future. He never guessed just how much power it gave him, for it gave him more power than a whole year at Hogwarts.
Meanwhile, the two wizard brothers in charge of transport had decided on using an uncontrolled cart. The cart would magically follow tracks to the castle, bringing the students to the castle at the start of the year. It would start at their hometown, London, and move through Oxford, Birmingham, Manchester, York and Newcastle before arriving at Hogsmeade station. This was the start of the Hogwarts express, and would become a train when they were invented. Due to this, the settlements at these locations would grow larger and more prosperous, including Hogsmeade. Soon more people would settle down in the mountainside village, shops would be set up and the village would be put on the map, were it not unplottable by muggles.
Soon, each section of the castle had been built. It had taken only a day, and the air shimmered with the high concentration of magic in the area. Everyone raised their wands and protected their sections from view, from attack and from the elements, making the outline of the sun wobble more than a fresh jelly. They all retreated into their tent, preparing for the big day ahead.
Early the next morning, the air crackled with raw magic once more. The sections were unveiled and all rose into the air, spinning as the main spell decided which way was the best to orientate each piece of this elaborate puzzle. Of course, the pieces didn't fit very well, so the spell filled in the gaps with extra corridors and extra classrooms, causing the castle to be bigger than was necessary, at least on the inside. Where pieces overlapped, they could be accessed different ways at different times. For example, the headmaster's office was located on the seventh floor, but could be accessed from the second floor or the sixth floor, depending on what time of year it was. Some staircases led to different places on a Friday, or at other quirky times. Even Harry's room could only be accessed if it was really needed.
Finally, Hogwarts was born.