Image credit: Eurosport
The Dutchwoman led the race from the first lap and finished an eventual 12 seconds ahead of Italy's Eva Lechner, who in turn was 26 seconds ahead of Kastelijn's compatriot Annemarie Worst.
"I felt really strong the last weeks, and this morning I felt really good," the champion said.
"I didn't like the course because it was really muddy yesterday and also this morning. I was so happy that it was dry in the afternoon.
"After the start I was sixth or something, and I felt really good. I felt like I was stronger than the other girls, and then when I had the lead it was full gas.
'Total Dutch dominance!' - Kastelijn takes European title
"It's cool to be in the front because you can ride your own lines. My coach said to do that, I did, and now I'm here.
"[I've completed] all my goals already from this season. It's incredible. Now I will focus on the Worlds, I think.
"It's really nice that I can race the whole year in this jersey. It's a dream – I'm really young and now my dream has come true."
Men's Final, 2016