The antonyms for the phrase "castle building" can be a range of things. Firstly, there are the physical antonyms associated with buildings, such as demolition, destruction, flattening, or dismantling. Secondly, there are different activities that are the opposite of "castle building," such as ignoring or neglecting, abandoning or disregarding, procrastinating or postponing, or mismanaging or failing to plan. Furthermore, the phrase can implicitly refer to daydreaming or planning for something that may never happen, so antonyms can include facing reality, being realistic, or taking action in the present. Overall, antonyms for "castle building" depend on the context and perspective from which the phrase is being used.