This mission is a challenge as well, mainly because of technical reasons, not so much because of the mission itself. The technical problem is that the saved-mission-file of Cossacks starts growing and growing over time and as I figured out can’t be loaded anymore after it exceeds a size of roughly 2.8 Megabytes. Because of this Cossacks might become instable and crash, and as I said: no way to load back the saved mission !
After a while I found out this is somehow related to the green Turkish units attacking from the NortWest via ships and ferries. After I took out a large gang just landed by ferry I realized that when saving at this point the file was shrinking.
So focus has to be put on those green friends right away and a couple of cannons will do the job to blast away their ships. This is a pre-requisite to really play and win that mission.
It is simply a matter of time to establish sufficient troops and build some mortars to first take out the green towers and then approach the blue Turkish opponent. Attack waves are weak so that the mission itself – as I said – is not that difficult. Once the blue army has been defeated the mission is won.
Besides using mortars there is another way to take out some of the towers – and this is a nice example of how easy it can be to fool artifical intelligence. Using a fast Sich Cossack works as well: just place him behind one of the towers and another tower close by will start firing at him – and hit the tower instead all the time.