The Castle is the final area in any run, regardless of paths taken you will end at the castle! So just playthrough a run beat the first 2 bosses and you'll be at the castle! A tip if the runs are daunting at first, you can use a save exploit to give yourself unlimited continues instead of reseeting on death. Everytime you press the Xbox logo button on your controller to bring up your menu, the game quicksaves. The quick save mechanic breaks when your character dies, so if you dashboard out quit the game and reload you will be at the last spot you hit the dashboard button. Pretty handy and makes getting to the castle a breeze! Good luck!