The Thirty Years War [Concluded]

[Ibn-Khaldun]

You could build a seige cannon, that would nulify the effect. or build a boat for transport then send your units out to sea.

If that is not possible, I would do temporary military access, with your units effectively being disarmed by dutch forces, then you are allowed to pick your weapons up when you leave. (aka, military access claus which makes you unable to use your units against me) Though, Ibn would have to say whether or not that is possible.

If worse comes for the worse, you could disband and I will pay the cost of you replacing your units. However, I am guessing that would require turns for you to replace, which you don't have at this moment.

If you have siege artillery and move your army onto hex adjacent to fort then siege battle will be fought. However, the losses for the attacker will be great since they are under constant artillery fire from the fort.
Also, an army with siege artillery automatically "besiege" any enemy fort nearby. Leave that army there and after 5 turns fort do not have any cannons left and it would be very easy to conquer it then(especially when it didn't had any garrison in it).

Edit: You can give him temporarily military access so that he could move his troops from your land.