Can someone tell me how to get into that treasure room on the main
entrance level of the castle? Also, where do you use this portcullis
key?

You can't get there from here. Or from anywhere. It's a fake.
The Portcullis key opens the portcullis, a handy shortcut. (It's a barry thing, almost like a jail-cell door.)

Yeah I figured out where you use that key, but are you sure you can't
get to all those treasure chests?

Quite, though there are 4 'deposit slips' which, in theory, which give you very nice items, theoretically from the treasury.

Oru

4 deposit slips? I think I only found 1 in the castle?

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Ya gotta kill the constable down with the drunks and steal the key
to his safe to get the other 3.

Or Pick his Pocket.

Thats what I did near the end of the game. My thief just stole everything. Pretty fun.