A fire on the castle's lower level

Halfway along the castle's lower level a fire blazes from floor to ceiling. Walking into it costs a life.
Nudge 1Where to look
What puts a fire out?
Nudge 2A stronger push
The water must be carried all the way up here. A bucket waits in a cave under the graveyard.
What you needOther problems to solve first
Solve these first (each opens its own page, no answers on the way):
Show the solutionThe whole answer, with a video
Bring the bucket full of water (A bucket in the cave) and drop it just before the fire: it's put out for good, and the rest of the lower level (four stars) and the way up to the castle's right side are open.