The Academy winger/defender will spend a second season continuing his development in his native Netherlands, having made 23 appearances for Ajax II in the Dutch second tier last term. He now steps up to the Eredivisie to play for a side competing in the Europa League this year.Castillo signed for Chelsea from Ajax in 2016 and has been a regular in our youth teams since. He represented our Under-19 side in the UEFA Youth League finals of 2018 and 2019, and more recently was in action for Andy Myers’ development squad in their league fixture away to Brighton.AZ Alkmaar were level on points with Ajax when the Dutch league was declared void earlier this year. They have drawn each of their three league games so far this season.