No tickets are required for the Citadel grounds or city fortifications, thus none can be purchased on site.
Since there is no entry fee for the **Citadel of Den Bosch** or for walking the **city's historical fortifications**, there are no tickets to be purchased, either online or on-site.
Visitors can freely access these locations during their operational hours (for the Citadel, access might be limited to daylight hours for security). If you are looking for paid activities, ensure you are not confusing these historical structures with other nearby museums or guided tours that might have a cost and require booking or ticket purchase.