Comabt Mission 15(carvin a path) seems to be unbeatable. Even on easy mode I fail every time. Googled some cheats but they don't seem to work with the gog.com version. Or maybe I'm just doing a mistake. Did anyone else try to use cheats??

I can tell you that cheats worked for me (pressing shift + alt + A in the menu, you might have to alter an .ini file) and that mission 15 is definitely doable. See the following walkthrough video if you need help beating it, I suggest you don't ruin the fun with cheats.
http://www.youtube.com/watch?v=0fwYF98H9uk

Thanks for your answer. I tried pressing shift + alt + A but nothing happens. Once I started the mission I pressed alt + Z to eliminate all enemy units, but it didn't have any effect.
I know this video, but still this mission seems to be very hard to beat. Thanks anyway.
cheers

Are you holding shift + alt + A down at the same time and are you doing it before you start the mission or during the mission?
Try starting the mission going to the main menu(esc) and then holding them down and then resume the game and try the cheat.

Your tunneliers seem to get ignored on mission 18 so it might work on 15 too. Other than that just move the archers forward slowly and kill the crossbowmen. It's slow as hell and my archers seem to be bugged most of the time but it works.
(I had to do the mission since cheats didn't work) ;)

uh uh! I know teh answer :3
To activate cheats you not only need to press shift + alt + A but also press proper cheats that you want to have able to activate during game :3
So:
step 1: Shift+Alt+A while in the Main Menu
step 2:
Alt+C - Access to all campaign missions at the selection screen
Alt+K - Free building (no resources required)
Alt+X - Add 100 popularity and 1000 gold
And remember - those are only cheats that works. ALT+Z belongs to cheats of early versions of Crusaders.
Btw - tunnelers glich, as far as I know, works only in mission 18 so I doubt it will work in 15 :3
Edit - this might help :)
Mission 15 walkthrough

So I just started playing Stronghold last week, and I've started work on a trainer. For me, at least half the fun in a game is exploring how it works internally (and how to break its rules ;] ) and I've been having a good time with SH.
So far I have written ASM patches for:
# Infinite gold
# Resource piles (wheat, stone, etc) don't shrink once they hit 10 in a pile
# Armory and granary stocks don't ever shrink
# (maybe) quicker population growth? Not sure about this one.
# Arrows do no damage
# Melee does no damage
Would anyone be interested in this? If so, I'll compile a trainer and post a link here.
-e

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Stronghold's single player missions are almost even impossible WITH cheats

I'm using Windows 7 64bit (just in case it matters) and I was able to get the cheats to work, but it took some experimenting. I was really only after one specific cheat, so your mileage may vary. I was trying to use the Alt-C cheat so enable all missions.

I started the game and skipped all logos and the intro movie. Then, when looking at the first menu screen I pushed Alt+shift+A then pressed Alt-C. I then clicked on military campaign which brought me to the next menu. Then, I again pressed Alt+Shift+A and then Alt-C. Then, I clicked the button which brings up the military missions and they were all available. Of course, I probably didn't have to use the code twice, but I was trying to be thorough. I had tried using Alt-C during gameplay (I had read elsewhere that it was the correct way) but it always failed.

Note on missions 15 (Clearing a Path) and 18 (Kill the Pig): these two missions become much easier when you use your crossbow men to pick the guards off the castle wall before trying to fill the moat. Your men have a 5% range bonus over the enemy. However, there seems to be a bug that makes your troops (or at least mine) shoot at the ground when engaging at the limit of their firing range. This bug is mentioned on non GOG forums so I believe that it is a problem with the original game. Without the range advantage, these missions are extremely difficult.

Everyone says this, but my tunnlers dont get ignored, they get killed just like everyone else. I've been trying this level for days now, and it's driving me insane :/

Thanks to the bugs(archers and crossbows shoot ground), in GOG version, I had to find a new way to beat the mission 15 other than using cheats. I filled the moat on the left side of the castle instead, and it WAS MUCH EASIER to brake in than from the front.

but I still dont see any solution for mission 18....crazy

mission 15 totally impossible because of that stupid bug (crossbow man shooting ground).
tried with several walkthroughs but still fail.
base10 if you are still here and have that trainer i would love to have it. thanks.

Yes the old strategies on the FAQs (like Gamefaqs) don't seem to work ... you definitely will not win an archer duel with them... one time I moved all my troops west, and was able to clear the moat enough to have my tunnelers bring down the walls protecting both areas, but it cost most of my spearmen/archers and but I did not have enough troops left to kill alll of theirs

I tried to use cheats and failed, so instead I modified the mission in the map editor. If anyone else wants to try this route, you'll need to rename the mission file (mission15.map) to something else before it's visible in the load map screen. You should also make a backup before you start editing.