Yeah, I just wanted to simplify my invasion. Its much easier to worry about 1 city than 2. Also, I captured horses for more keshiks.
I actually considered using the Khan to grab Cerro de Potosi for my puppet earlier. But I wanted the Khan's improved healing abilities for my army, and the combat bonus, which I judged as more important. I didn't want to use the khan for a citadel because I wanted to use the 2nd khan to get cerro and have the first for the army. Also, the land was flat, so could do 1 move forward, shoot, 1 move back just fine, even with enemy having great wall. Also, taking it slow was allowing me to bolster my keshik numbers, since i had focused so much on frigates and privateers instead of a land army.