Hello UA Darth,
5. If you occupy a space in your opponent's castle and you retract the drawbridge, it drops again on your next turn and not your opponent's, correct?
Yes, that's correct.
6. Is it correct that only ships can capture enemy ships? Land units can only commandeer them?
"To commandeer a piece" means "to capture a piece" though but if a land unit commandeers (=captures) a piece (vessel or camel) the opponent's piece will stay on board and it belongs to your units. Concerning vessels even if you disembark the captured vessel of your opponent afterwards.
In other cases captured pieces are removed from the board.
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