 Disguising himself as a cook and interpreter, he studied the traders , merchants and others who visited the castle, as well as the arrival and departure of ships.
<clarity>  Disguising himself as a cook and interpreter, he studied the traders , merchants and others who visited the castle, as well as the arrival and departure of ships.
 Disguising himself as a cook and interpreter, he studied the traders and merchants who visited the castle, as well as the arrival and departure of ships.