It was the .Evasolde castle (near Senderfall), and i have solved the issue (if you can speak about a solution)

I attacked the castle again, and again the siege tower was rollng, but after a while the tower stopped.
I notcied, you can push, and he moved indeed a little, but it doesnt work at the end, because the tower stopped again.

After a whle the enemy is sallying out the castle, and i was fighting them on the field before the castle. After a while i (my character) was defeated on the field.
But then, immediately after i was defeated, the siege tower was rolling again and reached the castle.

I let my army siege the castle, and take over the castle. Mission accomplished :)