Sometimes barrels roll off the screen before reaching the bottom when you are nearing the top level. There can only be one barrel on any ramp at one time. Use the hammer on both screens to get the most points. On higher levels, stand by the ladder under the girl to get the barrels as they fall. There are some glitches on the second screen that appear.
Fireballs can jump rivet holes at any time.
This usually frees it up. A neat trick is to trap fireballs on the outer girder sections by jumping over them and clearing a rivet at the same time. On your way out of the door, after collecting treasures, get hit on the legs with a fireball. It apparently hurts the second player more than you. When trying to enter the castle, watch for the pattern of fireballs that come out of the door.