"(I visualize Hogwarts as) a huge, rambling, quite scary-looking castle,
with a jumble of towers and battlements. Like the
Weasleys' house, it isn't
a building that Muggles could build, because it is supported by magic."
-- J.K. Rowling (Sch1)
Hogwarts Castle is a wondrous, magical building with many towers and turrets. It is home to Hogwarts School of Witchcraft and Wizardry. If Muggles approach the castle, all they will see is a mouldering ruin with a sign warning them to keep out, that it's unsafe. Hogwarts is built in a forested area, directly next to a fairly large lake, in the highlands of Scotland. The main entrance to Hogwarts is a set of huge oak doors which face more or less toward the west . The lake is to the south of the castle . The forest is very large, and parts of it extend around to the west of the castle .
What can we say about Hogwarts castle? What does it look like? Well, for one thing, it's huge. Seven stories tall is utterly massive for the main building of a castle. This is especially true when you consider that castles of the stone wall type were not even invented yet in the 900s. Some have speculated that wizards invented castles and that Muggles copied them. I think it's more likely that Hogwarts school has changed in appearance quite dramatically over the years. As originally constructed, Hogwarts was likely much smaller, perhaps about the size of a manor house. As the years went by and as student populations grew, the school building expanded and adjusted for the changes. We do see modern plumbing in the castle, for example, and this surely wasn't put in place a thousand years ago. Rooms were added (or possibly they simply "grew" as needed) and the whole thing became something of a warren of passages, corridors, rooms, doorways, and staircases. Many of the secret passageways probably evolved as rooms, hallways, and even whole floors and wings added themselves on, leaving spaces and gaps.
On the whole, then, Hogwarts is likely a rambling, complicated, diverse combination of buildings of varying styles and architecture. It's held together by magic, so the towers literally defy gravity and the various types of construction fit together in impossible ways--wooden beams join seamlessly to stone battlements and plaster walls meld into iron door frames. And a "floor" wouldn't be exactly the same everywhere. The first floor near the marble staircase might merge into the second as it connects to another section of the building where the floors don't match up. The third floor, if you traced a line parallel to the ground across, might meet up with the second floor across the courtyard.
On top of all that, the various rooms and spaces in the castle tend to move around, according to Rowling . The Famous Wizard cards attribute this magical, rearranging floorplan to Rowena Ravenclaw. This can't be happening all the time, surely, or no one would be able to get anywhere, but it does happen enough to keep people on their toes. There is no example of actual moving staircases in the castle (that's a movie invention, although there is mention in OP of a "swiveling staircase" which could be similar to what we see in the first film) but it is clear that things do change. The Hospital Wing is a good example; it moved from the first floor in 1992 to the third floor in 1996.
No single, static floorplan is possible, then. However, since most things do stay in the same place throughout the series, it is possible to make a list of what's on each floor. The actual internal arrangements of each floor can only be determined for small sections of the castle at a time, and are usually of very little interest (a corridor with a statue of a wizard and a room to the left, for example). The Lexicon includes a map of the Ground Floor, however, showing all the rooms which can be positively located, and maps of the Gryffindor boys' dormitory and the castle and grounds.
Explore Hogwarts Castle and Grounds:
 This is probably for Rowling's convenience more than anything else, since it gives her an automatic "out" if she makes an error in placing the same room in different locations from one book to another. There aren't a lot of examples of actual errors of this kind, though. Moaning Myrtle's bathroom is really the only serious mistake--it moves between floors from one chapter to the next. Just about every other room can be located fairly precisely and hasn't appeared to move much. True, the Hospital Wing appears to have jumped upwards two floors, but that was between several books and could have been relocated intentionally. Also, since it's a "wing," not simply a single room, it may extend over several floors.