Why is it so difficult? (Cossacks 3)

There is a problem in the second mission the Holy League. Can't get through - difficult to impossible.
Have you encountered any problems with this mission? Just don't have time to create a sufficient number of troops to attack, and to restrain the attacks of the enemy.
How to capture the fortress in the North do not understand - the road is blocked.
4 hours on a mission was killed.