The dress code for the ceremony is Cocktail or Garden Attire. Weather permitting there are portions of the day where we expect to be outdoors on grass, so for guests so inclined to wear heels we would advise against anything prone to sink into the grass.
The wedding is in the province of South Holland, so this is technically true. However, when people use the terms "Holland" and "The Netherlands" interchangeably, they are often incorrect. The Netherlands has 12 provinces, and only two of those are "North Holland" and "South Holland".