William's dramatic entrance had been preceded by an excruciatingly protracted journey across the roof, one which involved a great deal of crawling, jumping, and sliding, with at least two instances of nearly slipping off the side. There was a reason they didn't find kids up here very often, he'd decided. Not many people were foolish enough to use it as a pathway.
The Observatory, as the Ravenclaws called it, was cobbled together from stone and magic on the outside of the tower, near the top where there was a clear view of the sky in all directions. It was essentially a balcony that could be reached through two doors, both leading from the lavatories. The Astronomy Tower couldn't be reached after curfew, and for a House full of students who did homework at all hours of the morning and waited up past midnight to watch celestial events in action, that was an unacceptable limitation. So they used their own.
The doors to the Observatory were kept locked to prevent sleepwalking mishaps. There was a password, of course. Unfortunately, dangling from the lip of the Tower with his wand tucked securely into his waistband and holding tight with both hands, William had had no way of casting it. The next logical thing to do had been to kick the door in.
Which was how he ended up blasted with a Stunning spell the second he slid into the Tower - possibly he should have expected that - and toppled onto the lavatory tiles much less gracefully than he had originally intended.