No doubt theyve heard the stories of the Mother of Dragons and how she left a smoky ruin where there was once Yunkai and Astapor. No doubt too, they believe as the Yunkaii did, that they are not like the other cities; they are different, better, somehow more able to resist. The rulers of Meereen make the same fatal mistake of thinking that Daenerys wants their slaves for herself, but as they are horrified to discover, she is there to set them free, and turn them against their masters. The Meereenese also embarrass themselves by sending out a champion warrior to fight a champion from Daenerys side. What was the point of that? No terms were discussed as to the actions of each party at the conclusion of the duel. Are they assuming that she knows their customs and would just pack up and leave if her man lost? If theyve learnt anything from Astapor and Yunkai before them, which they clearly havent, its that Daenerys does not fight fair. If they thought they would gain anything from this challenge other than an attack on their city, then they probably deserve to be conquered.
