Well changing the variable type to SpaceWorld did it :D (thanks, I think that will help lots with my troubles with references!). However it doesn't like:
it doesn't like castle and says "cannot find symbol - variable castle". Aren't we defining the variable right now though? Or is it because there is no variable type given? Or am I just rambling?
private void prepare() { castle = new Castle(); addObject(castle, 300, 399); }