Lotto Soudal's Andre Greipel took his second win in the Sterm ZLM Toer.
Conor McConvey and Lars Boom were early attackers before British rider Scott Thwaites took a third place on the Cauberg climb in a 190.5km stage which started and finished in Buchten.
At the intermediate sprint at Bemelerberg, time trial winner, Roger Kluge, who was now part of a ten man break, took the points and was part of a group which was 3.15 ahead when Castelijns was first over the Loorberg climb.
Gianni Meersman took the next sprint and climb as the gap went out to 4.05 with 86 kilometres gone.
With fifty kilometres to go, the lead was down to 1.40, and still 1.25 twenty five kilometres later.
Iijo Keisse took the next sprint with points for Jetse Bol which put him in the lead of the competition.
Keisse, Meersman and Roy Curvers, tried to stay away but they were caught with four kilometres to go and Andre Greipel won the mass sprint in 4.29.21 ahead of Moreno Hofland (LottoNl-Jumbo) and Edward Theuns (Topsport Vlaanderen Baloise).
Marcel Siberg said to the Lotto Soudal team press office about Greipel's victory: "There was headwind in the final. Lars Bak led us to a half kilometers from the finish. Then I took over. Afterwards it was the turn of Jens Debusschere and Greg Henderson. Andre was a little tired at the end of the ride, but could still finish narrowly.
"It took almost an hour for a leading group was driven away. There was one of them close in the standings, so we could not give them too much space. Dennis Vanendert sat immediately in front, then came Sean De Bie who chased with Lotto NL - Jumbo & Trek, later lending a hand.
"We were able to catch the escapers in time and let Andre off the reins so he could win and we could celebrate.
"Two weeks before the Tour, it's nice to win here twice. Tomorrow is a tough ride in the Ardennes, tonight I already drink a Belgian beer to celebrate the victory (laughs). "