Hogwarts was a very large castle, so large in fact, that even those who had a map could get lost (it was a wonder how Filch got around without one.) It was for this reason that a few Gryffindor students had decided to create a map of the school so they would know where they were and where they were going. One night while out scouting the castle to get the layout, they got lost. Though they wouldn't admit they were lost, they were. In later incidents, they would simply say they knew where they had been- which wasn't exactly a lie. They knew they had been in Scotland, somewhere in Hogwarts. Later on they would come to find they were in the Western Tower.
The Western Tower of Hogwarts was deserted long ago, and was simply there because it was. The classrooms were old and dusty, crippling with eons of age. Nobody ever went into the tower, even though Professor Dumbledore had never advised against it. The tower had no purpose to anyone- except The Marauders.
When Sirius Black had sneezed and flew back into a bookcase that night they were lost, they were jovial to find the bookcase was actually a door that lead to a room as large as their Transfiguration classroom. (Sirius didn't really care to begin with, he was under a pile of books that had flown off of the shelf he had smacked into and was in need of medical assistance.) They revisted every night for the following two weeks, finding hidden passages to other rooms that were linked to the deserted classroom, and each claimed one for whatever their purpose was.
After they had cleaned up the classroom well enough, spiffied it up a bit, and sealed their own rooms with passwords only they knew, they spread the word through the student body about the room up in the Western Tower- and held parties there every Friday and Saturday night. Though it took a while for the parties to pick up (and for the Slytherins to show up- they hated it because the Gryffindors had found it- but couldn't find anything better to do on Friday and Saturday nights- though the Gryffindors heartily argued that they didn't necessarily care if the Slytherins showed up...), and they were a great success by the middle of the year.
The teachers suspected something at once, but could never prove a thing. It just seemed that the students had become very interested in their homework- especially on weekends- and adjourned to their dorms early. This thrilled the teachers to no extent, and they found no need to investigate. And once James Potter and Lily Evans became Head Boy and Head Girl, they helped mask that there was indeed something going on up in the Western Tower (which Filch highly suspected but could never prove.)
After the Marauders left, however, the Head Girl (no longer Lily, who had covered up for them and had went to the parties- this new Head Girl was a tattle-tale and had never been invited to the parties because of this) found out and told Filch about it. Filch sealed the room up, and all the entrances into the Western Tower. The students were disappointed horribly, but still told the tale to the first years of the room up in the Western Tower where The Marauders had held parties. Soon the tales were forgotten, however, and the room was just a whisper that nobody cared to listen to. The Western Tower was an enigma that nobody cared to solve.
But years and years later, the tower was reopened...