Towards the end of the story, if you've chosen to help Bartolomeu earlier and are now involved with Henry Morgan's scheme to steal the Aztec treasure, your part in the assault on the El Morro fort is to sneak around a large field, killing the guards before blowing up the cannons. You have to kill all the guards without any of them being alerted and running off to sound an alarm. To this end, you're equipped with poisoned throwing knives. These take a long time to kill a guard and for a while I thought they didn't work at all. So I tried throwing cobblestones at them instead. This knocks the guard out cold, then I can go up to him and finish him off with the sword. After I'd taken out all the guards this way, the red flashing indicating a battle in progress was still there, the quest book did not update and I wasn't able to proceed to blowing up cannons. Searching the whole area didn't turn up any guards I'd missed. I'd saved game just after being teleported to the field and before trying to attack anyone, so I reloaded that. Plan B was to simply run up to a guard and hit him repeatedly with a sword to kill him before he could hit me back or run away, and that actually worked. I killed all the guards, the quest book updated, and I was able to continue on to setting bombs. Reloading the game and trying again with cobblestones gave the same result as before - the game didn't recognise that all the guards were dead. Both forms of attack were repeated with consistent results - if the guards were first stunned with cobblestones then the fight didn't end, if I just charged and sworded them then it did. It turned out that poisoned throwing knives do work, eventually. I threw one, left the computer while I went to make a cup of coffee, and returned just in time for the guard to fall over. Not wanting to spend all night taking guards out that way, I ran round and sworded them all again, then continued with the rest of the game.