Trichocereus terscheckii, Argentine Saguaro Cactus
Trichocereus terscheckii (Argentine Saguaro Cactus) grow to an astonishing 30 + feet (a Trichocereus in Saguaro clothing). In fact, the Trichocereus terscheckii are sometimes called Argentine Saguaro.
If you need to batter-ram down the castle doors, Argentine Saguaro Cactus should work just fine and probably wouldnt hurt it a bit. Trichocereus terscheckii are the heavyweight champs of the Trichocereus; huge and solid (a 1 foot cactus weighs over 20 lbs and is nearly 1 foot thick).