One cannot buy tickets for Central Terschelling Castle as it does not exist. For actual attractions like museums on Terschelling, tickets can typically be purchased on-site at the entrance.
Since 'Central Terschelling Castle' is not a real attraction, there are no tickets to purchase, either online or on-site. For legitimate historical attractions on Terschelling, the common practice is to allow on-site ticket purchases.
For example, visitors to Museum 't Behouden Huys can buy their admission tickets directly at the museum's entrance. Similarly, for any special events or temporary exhibitions at other cultural sites, tickets are usually available at the venue's reception desk. Advance booking might be recommended during peak season or for specific guided tours, but for general admission to most sites, on-site purchase is standard.