Yes, tickets for climbing Lange Jan tower can be purchased on site at the entrance to the tower.
Yes, tickets for climbing the Lange Jan tower, which is the main chargeable activity associated with the 'Old Middelburg Cathedral' sites, can be purchased on site. There is usually a ticket booth or counter located at the base or entrance of the tower where you can buy admission tickets directly.
For general access to the abbey complex and the Nieuwe Kerk interior (where possible without a tour), no tickets are required. It is advisable to check opening hours for the ticket counter, especially if visiting outside peak season, to ensure it is operational.