I'm pretty sure you only need to capture the king to win. Capturing the queen is just convenient because then they're down the piece that can merrily fly across the entire board as long as there's nothing in the way (and if there is anything in the way there soon won't be XD).

LoL never underestimate the simple survival tactic of being ignored XD

I think the rooks/castles were supposed to be watch towers rather than actual castles (as far as the piece representation goes) but I think they were originally chariots or something (far distant memory from when I was actually interested so I could be wrong/mixing it up with something else).

I didn't actually know that about the king and queen pieces O_O