by louper
The castle allows you to IMMEDIATELY carry out an additional action, not "do something else and then carry it out."The castle does grant you an additional action; note that the rules say you can buy from the Depot before, between, or after your two DICE actions. The action granted by the castle is not one of your two DICE actions.
Both of these interpretations support the fact that you cannot buy something between placing a castle and using the action granted by the castle.