The Lost Skull Achievement 'Horses Can't Swim' reads "Complete 'The Lost Skull' without using any cavalry". Once you click on the achievement and put your cursor over it, it reads...Complete The Lost Skull using only the following units: Recruits, Militia, Bowman, Cavalry, Longbowman, Elite soldier, Crossbowman, Cannoneer. As you can see whats missing is soldiers. So i completed it without reading the text carefully and used no cavalry and the achievement did not complete...so it should read "Complete 'The Lost Skull' without using any soldiers". This typo waste my time. I don't know if this is a known issue or not just wanted you guys to know this before doing this adventure.
update: Redid adventure with no soldiers got the achievement.