We use not to give survey

Of our chief strengths to strangers; our citadels 

Are plac'd conspicuous to outward view

On promonts' tops, but within are secrets. (I.i)

    For me these lines resonate more than any other lines in Act I.  The act of concealment discussed in these lines echoes in Deflores's love for and plotting to possess Beatrice and in Antonio's playing of a fool.  Furthermore, Vermandero's words remind us of the title of the play, The Changeling.  Although little is known about Antonio, named the changeling in the dramatis personae, it is suggested in Act I that his changing involves deception and secrets similar to guarding of secrets in the quote above.

(sorry, I don't have the text yet and have been reading online where there aren't any line numbers)