I have a plan that destroys his castle as well. 

Its called letting Reno go over there and having him use Meteoga. 

Boom, problem solved. (Even Joshua, the character, has said, or thought, that Reno could destroy a city if he puts his mind to it).

Or if the castle is made of candy, just let Reno go over there and he'll eat it. 

That also solves the problem XD