As "Royal Zwolle Castle" does not exist, there is no best time to visit. For other Zwolle attractions, spring and early autumn offer pleasant weather and fewer crowds.
Given that "Royal Zwolle Castle" is not an actual attraction, there is no best time to visit it.
For visiting Zwolle and its existing attractions, the spring (April to June) and early autumn (September to October) are generally considered ideal. During these periods, the weather is often mild and pleasant, suitable for exploring the city on foot or by bicycle. There are also typically fewer tourists compared to the peak summer months of July and August.
During the summer, Zwolle can be lively with outdoor events and festivals, but also more crowded. The winter months can be cold, but the city often has a festive atmosphere around Christmas markets. Specific attractions, like museums, might be less busy on weekdays compared to weekends.