I'm reading a great book called the "History of Venice" and what you all are describing seems to be fairly accurately gauged against the Fourth Crusade. The Venetian navy was paid (though not in full) to provide naval transport and troops for a sack of Constantinople. Upon success, Venice planted pedosa around the outskirts of the city and also held something around 1/3 of the city's quarters for their own. The sack of Constantinople sents ripples throughout the empire but would eventually cause more problems for western powers then they had hoped.

And, yes, I am playing as Venice in my campaign. :)