The Siege Perilous is achieved by completely closing the toilet seat, then shitting onto the lid. Although not technically difficult to accomplish, this prank requires a high degree of stealth, not only to enter the bathroom undetected, but also to affect an expeditious retreat. Ideally, you don't even want anybody to know you're in the bathroom when you attempt this maneuver, and you certainly don't want to be detected exiting the now-tainted chamber! As such, this prank is not recommended for those with weak constitutions or disorders of the nervous variety.
Just as the celebrated Knights of the Round Table trained and prepared for their quests, so too must you, young squire. Do not even attempt this prank unless the house is full of people. This may seem counterintuitive, but remember: the more guests present, the more diffuse the blame. Short of compelling eyewitness testimony, the host will be unlikely to blame any one person in particular -- so make certain you are not seen entering or exiting the bathroom!
It is also wise to ensure that your previously-planned escape route remains viable. You may wish to make a "dry run" before actually attempting The Siege Perilous. While it may be acceptable to retreat via the bathroom window, ask yourself, where is the chivalrous honor in running like a cretin through the night? Far better it is to sneak into the lavatory, commit the act, then calmly saunter back through the house, disappearing into the crowd like a fabulous spectre. This will also deflect suspicion toward whichever guest happens to leave between the commission of The Siege Perilous and it's awesome and terrible discovery.
The power of The Siege Perilous is twofold. First off, upon initial discovery, there is an almost magical moment where the world stops, the mind languishes in a state of cognitive dissonance, and things that were once commonplace now seem tinged with an almost alien peculiarity. Once the observer is able to truly comprehend what has happened, a second mental state begins to emerge. Rational behavior would dictate that the crap on top of the seat should be disposed of by flushing it down the toilet. But, given the placement of the poop that forms The Siege Perilous, how exactly is this to be accomplished?