While the Hyundai A-League will be new territory for the 31-year-old, there will be some familiarity for Castelen as he has played against current Wanderer Matthew Spiranovic while the pair was in Germany.
The speedy winger, who plied his trade for Hamburg at the time, was also a former team mate of one of Western Sydney's proudest exports Brett Holman at Dutch heavyweights Feyenoord along with former Wanderers hero Shinji Ono in 2004/05.
With a pedigree that includes a successful World Cup Qualifying campaign with the Netherlands as well as experience in the UEFA Europa League, Castelen will be an invaluable asset to the squad as the Red & Black prepare for the upcoming Hyundai A-League, AFC Champions League and Westfield FFA Cup campaigns.
Wanderers CEO John Tsatsimas said that Castelen was a perfect addition to the Club.
"Romeo is coming here fresh from a season in the Dutch first division where he was a standout for his Club. He's a creative, attacking player and we look forward to him bringing that form to the Wanderers," said Tsatsimas.
"His 10 national team caps for the Netherlands speak for themselves in terms of his ability. Romeo is a great talent who will make a seamless transition to A-League football.
"It is players of this quality that we look to bring to the Wanderers. Having Romeo on board will raise the standard of our squad and that's what we want out of a new signing."
Castelen earned his 10 international caps playing alongside the likes of Robin van Persie, Klaas-Jan Huntelaar and Phillip Cocu, starting in the majority of matches he played in under coach Marco van Basten.
"I am very happy to sign for the Wanderers. When I spoke to the coach (Popovic) he convinced me very quickly on the project," said Castelen.
"I am excited by the new opportunity. The Wanderers is a great Club, they have a great coach and a big fanbase. I like to play in front of big crowds and I looked on the internet and saw how passionate the fans are. I am very much looking forward to being a part of the growth of the Wanderers.
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