The Surinam-born Dutchman was in outstanding form for the Wanderers in their 2-0 win over Melbourne Victory that was easily the best and most entertaining much of the season.
Castelen gave hapless Victory fullback Daniel Georgievski a torrid time on the right wing and it was fitting that he was given a rousing ovation by the 17,000 crowd when he was replaced with six minutes to go.
Castelen made the first goal for Mark Bridge early in the second half and sealed the Wanderers' victory with a goal of his own 11 minutes from time to cap a memorable performance.
The striker joined the Wanderers from Dutch club RKC Waalwijk in 2014 but his first season in Australia was interrupted by injuries.
However after a strong pre-season Castelen is flying again.
"The main thing that I'm focussing on is to stay fit and play games because I know my ability and the good things will then come," he said later.
"Last season was tough also because of all the travel we had to do.
"We are playing a totally different system this year and it is enjoyable to be part of this team.
"We play a possession based game where attack is the main focus.
"If you have the ball the other team cannot score so it is less running defensively.
"We press very high and that is very good for me because now we win the ball up higher and we can be more dangerous so it is very pleasing to play in such a system.
"The boys did fantastic today and when we went back to the dressing-room we all said how unbelievably well we pressed them.
"I think Besart Berisha had no space to breathe because he had three, four defenders around him.
"All over the pitch teammates were backing each other up.
"That was the key today. We were very aggressive when we did not have the ball and we kept our composure when we had it. It was a great win today."
Castelen said he was pleased to note that the team was turning its strong performances into results.
This was the Wanderers' seventh straight victory that kept Tony Popovic's team at the top of the ladder after 10 rounds.
"We started the league with a home defeat to Brisbane but the coach rams into us every day that we can beat any team," he said.
"We have always had the belief because we train every day and we see what we can do.
"Playing well and not winning makes you lose your belief but now the results are coming.
"The scoresheets are matching our performance. We've beaten a good side today but we have to keep improving.
"I think we have to give Victory respect. They are a good team with great players."
Castelen's positive sentiments were echoed by Popovic.
"The picture is clear. Today's match showed we can compete with anyone," the coach said.
"If we had Victory in the first part of the year maybe we would not have been ready for them.
"But the players are a lot fitter now and fluency and understanding are better so it was a right time for us to play them.
"So today showed that if we play well Victory are a team we can beat."