1) What is filled in the morning and emptied at night, except for one night of the year when it is filled at night and emptied in the morning?
2) What is impossible to hold for half an hour, yet weighs virtually nothing?
3) What demands an answer, but asks no question?
4) I've little strength, but mighty powers;
I guard small hovels and great towers.
But if per chance my master leaves,
He must ensure he safeguards me.
What am I?