Instead of opening the door like an idiot, Castiel hefts his knife and throws it through one of the sigils he'd scrawled on the door in his blood. The response was immediate; the Leviathan -thankfully, one of the dumber ones- burst in and Castiel filled it with an explosive spray of pellets.

Not that it did any good.

The Leviathan pursues Castiel with single-minded purpose and it gives Dean the perfect opportunity to slow it down.