Here is the full list of all 40 Defend The Highlands achievements.
Kill 1,000 enemies
Fully upgrade a stat
Have 50 Scotsmen at once
Complete a skirmish match on impossible
Complete a skirmish match on hard
Kill 500 Irish Potato Farmers
Complete a skirmish match on medium
Recruit 100 Scotsmen in total
Complete a skirmish match on easy
Complete a skirmish match without heroes
Complete a skirmish match without towers
Kill 100 enemies
Kill 10,000 enemies
Kill 1,000 enemies with porridge bowls
Kill 500 enemies in melee
Kill 1,000 enemies with porridge cannons
Kill 500 enemies with whisky vomit
Kill 1,000 enemies with golf balls
Kill 1,000 enemies with haggis catapults
Kill 250 enemies with bagpipe artillery
Kill 1,000 enemies with turnip turrets
Kill 500 enemies with cheese wheels
Recruit 1,000 Scotsmen in total
Get 500 kills with Alfredo
Get 500 kills with William Wallace
Get 500 kills with Robin Hood
Get 500 kills with Queen Boadicea
Kill 500 English Cricket Players
Kill 500 Welsh Rugby Players
Kill 500 Leprechauns
Kill 500 Ancient Briton Warriors
Kill 500 Posh Englishmen
Kill 500 Welsh Sheep Riders
Kill 500 Beefeaters
Hit 5 enemies with a single artillery strike or haggis
Kill 15 enemies with Nessy in a single match
Fully upgrade all stats on a single unit
Defeat a level 6 enemy on level 1 in skirmish mode
Have 5,000 porridge at once
Have 2,000 gold at once