Okay I try to explain it.
I select my hussars and then I order them to destroy the palling. If i don't order themt immediately the game deselects the hussars and selects the six cannons. The cannons will destroy the palling, because they were the selected units when I ordered destroying the palling, but I wanted the hussars to do this.
The hussars and the palling are just examples.

It is hard to explain well, so I attached the save files. One before capturing the six cannons and one after capturng them.

(The game will crash unless you install the campaigns and make a player profile first.)

Here are the save files: https://www.mediafire.com/?kcnskie23cmec7e

I don't know if this problem exists in European Wars, but I will check it.

Edit
It exists in European Wars, too. It is not a critical bug, you can complete the mission. I think we should leave it aside.

I discovered a bug while I was playing the mission in European Wars. In European Wars when the enemy destroys the city which was under siege, you get this message "You are in a tough situation..."
Screenshot: http://s10.postimg.org/boct3ysxk/Message.jpg

In Back to War you don't get this message, because a lot of riders and some dragoons don't go to the siege.
Screenshot: http://s10.postimg.org/goeupnny0/Riders.jpg

It's not a critical bug either, but it's strange that the bug only exists in Back to War.

For the first bug about the cannons: You're never supposed to capture the cannons if you play the mission normally. The only way you can do that is if you use cheats or something happens to the enemy troops, because the mission is scripted so that the black player loses the battle and the city falls, forcing you to run north.

Where did you get the hussars from?

They get selected because that's how the script works, since you are not supposed to capture them you should not run into any problems unless you use cheats. A trigger for player blue is causing the selection.

As for the script itself I noticed as well that BTW's script is buggy, perhaps more so than AOW, especially with selecting and sending units via scripts (SelSendTo and SelSendAndKill, SelectUnits etc.). I'm beginning to regret I did not script my campaign for AoW insted of BTW.

Ah, well....

That is all I can tell you without seeing the mission's source code. I would really like if the developers would release the full source for the mission scripting...

No, I didn't use cheats.

You can liberate the priest (priest Buisonis) and complete the mission without any cheat codes, right?
What happens if you go the North-West before liberating the priest?

There is a Town Hall and a Storehouse controlled by the blue player in North-West. You can capture them and train peasants. You can make them gather wood, stone. You can build a mill and mines. You have billions of food, coal and iron from the beginning of the mission, you can trade them and buy gold, wood and stone. If you have a lot resources and some peasants, you can easily build all the buildings and progress to the 18th century.

I think you can guess the remaining steps.

Very interesting. I do not believe the developers intended that at all.

You mean go north west before liberating the second guy, the one in the empty fortress right?

You went past the towers and enemy musketeers?

PS: What did you use for screenshots, my fraps has issues with capturing cossacks.

Yes, I mean go north west before liberating the second guy.
I went past the towers and met with a lot of pikemen, they surrendered.
You can kill the enemy musketeers with them.

(I think the gate won't be opened if you don't kill the musketeers.)

I used the printscreen button on the keyboard and pasted the screenshot in Paint.
(You can only make one screenshot at a time, but it works.)

By the way, I usually use FastStone Capture or Steam to make screenshots, but they don't work with Cossacks.

Umm...am I the only one that thinks that you should be getting paid a percentage for this?

Some people say they did it before me, anyway, it doesn't matter. I do this for fun, and the point is, I released the same thing without the bug, and for free for all to enjoy, since most people already own all three games.