Atlas of Hogwarts

The Marauder's Map

The Marauder's Map looks like a blank sheet of parchment until it is activated. Then a wondrous map appears, detailing the interior and exterior of Hogwarts castle and showing the people inside the castle as miniscule figures, labeled with their names.

How does it work?
In order for the two-dimensional map to show a three-dimensional castle, some magical viewing effect must take place. We don't know exactly what that is, but perhaps it is similar to looking at a hologram. As the viewer focuses on various areas of the image, those areas come into focus and extremely tiny details can be seen. It would seem that things appear only when there's a reason for them to appear. That's why the little figure of Harry only appeared on the map when he took possession of it and needed to know how to work the secret doorway.

Mysteries and questions about the map
How could four underage wizards create so powerful a magic item?
Where is it now?
Why didn't Scabbers or the time-traveling Harry and Hermione show up on it and be found out?
How did Fred and George figure out the magic words with no clues or help at all?

Activating/Deactivating the Map
In order to activate the map, a wizard must touch the map with his or her wand and say "I solemnly swear that I am up to no good." The map will then appear. To deactivate it, the words are "Mischief managed," upon which the map goes blank.

Orientation of the castle
We don't know the exact orientation of the castle. We do know, however, the following facts:

the lake is to the south of the castle
the forest is located to the west of the castle (although it is very likely so large that it surrounds much of the grounds) since the sun is seen setting over the forest in PA
if the movie is to be believed, the castle is approached by the lake from the west, since it is early evening and the full moon is not high in the sky (the full moon always rises in the east, opposite the setting sun)
the front doors face more to the west than the east, since the setting sun casts beams of lights through them in PA

Hidden passageways
Like any self-respecting castle, Hogwarts has plenty of hidden passageways. There are seven shown on the map which lead out of the castle, including one which originates behind the statue of a hump-backed witch on the third floor and another behind a statue of Gregory the Smarmy. There is also a hidden passageway which connects the Whomping Willow to somewhere off the map. Two of the passages are caved in.

Other powers of the map
The Marauder's Map apparently has a sense of humor. When Snape used various magical phrases to try to induce the map to reveal its secrets, it instead insulted him by name. It is unclear whether the map was specifically "taught" to recognize Snape, who was a contemporary and enemy of the creators of the map, or if the map is actually imbued with a level of intelligence and personality.