No, it is impossible to buy tickets on site for 'Open Roosendaal Castle' because it does not exist.
Ticket purchases, whether on-site or online, are only applicable to genuine attractions. As 'Open Roosendaal Castle' is a fictional location, there are no tickets for sale, and thus no on-site box office or entrance. Any attempt to purchase tickets for such a place would be futile. Visitors should always confirm the existence and legitimacy of an attraction through official tourism channels before making travel or ticketing plans.