[Taqwus]

A Star Child could assassinate the Monolith with a mind duel (already astral-1; starshine cap makes it 2, crystal coin if R'lyeh can get one makes it 3) so long as the castle's not already under siege. However, first it has to get there, and other than R'lyeh astral assassins are rare. IKerensky has it right on the danger from R'lyeh.
Other than that, mind duel is an unlikely threat. A Monolith is too big to Wind Ride, and the only remote magic-duel is from Strands of Arcane Power, which would require the caster's dominion to have overpowered your own. Again, if that happens, you've got bigger problems.
(Ok. An Elludian Moon Mage with a black heart could do it, but the Hidden Kingdom of Elludia is a rare sight, *and* for it to be found by a blood-happy nation isn't too likely.)