In the second part of the castle aftr you beat the giant bat. Go up the stairs and on the top stay there for a few seconds and a thing will appear on the bottom get it. Or I think you can go up the stairs, break the wall, stay there and something will appear on the bottom.
On the level with the spikes, go to the bottom of the steps and attack it. There will be a peaice of chicken there if you need it
When you are about to fight the giant bat, go under the stairs and jump and attack the ones that are in mid air, there will be a roman numeral, that means you can throw more than one sub-weapon at a time.