Attacking a castle has long proven difficult, what with the moats, high turrets, and Nathan Fillion's surprising field-readiness despite not actually being a cop. Giving you the most effective measure to demolish the first kind of castle, the Trebuchet Store
From a US Navy Weapons Systems Engineer whose interest in medieval warfare had him building 'pults by age 11, TS's a Swansea-based online repository of good-natured destruction that peddles kits/ models/ DIY blueprints for building trebuchets, which're quite obviously 12thC gravity-powered catapult/slingshot hybrids notorious for destroying castles with "precise and consistent projectile delivery" -- something you used to also be able to experience by eating downstairs at the Kell's. Pre-built ready-to-fire/assemble kits include the powerful 32in-tall Stirling that can hurl an object over 80ft; the indoor/outdoor Da Vinci, which boasts an open counterweight cabinet for easy "range and trajectory adjustment"; and a red oak/maple tabletop piece called the War Wolf, also what Mr. Blitzer's co-anchor whispers to him whenever he loses his train of thought. There's also an extensive lineup of step-by-step DIY plans (that use easy-to-find materials and common tools) for a 12in Onager catapult, once the most powerful weapon in the Roman arsenal with a "kick" that resembled a Persian donkey; its sling-less predecessor, the classic torsion Mangonel; and the tour-de-force, an early crossbow-like Roman ballista missile projector that can throw stones over 90ft, but really shouldn't, since it just bought that glass duplex in Arlington
In case you prefer less intense shellings, TS peddles more prank-friendly options like a six-foot tall tennis/racquet ball hurler, as well as one made of PVC pipe specifically designed to launch golf balls, something Castle hasn't quite mastered, as that dude hails from cold-ass Alberta, Canada.