As "Royal Zwolle Castle" is not a real attraction, ticket purchases are not applicable. For other sites like Museum de Fundatie, tickets can be bought on-site.
Since "Royal Zwolle Castle" does not exist as an attraction, the question of buying tickets on site is not applicable.
For actual attractions in Zwolle, such as Museum de Fundatie, you can typically purchase tickets at the entrance. However, many museums and popular sites now encourage or require online ticket purchases in advance to manage crowd flow and ensure availability, especially during peak seasons or for specific time slots.
Checking the official website of any intended attraction is always the best way to determine their current ticketing policies and whether on-site purchases are possible or if advance booking is recommended.