Plat the "A Murder of Crows" level, where you have to escape from the prison camp. Instead of following the Deceiver south after you defeat Soulblighter, follow the archer's advice when he said "hey, we saw a wagon go north a few days ago". Exit the level north of Soulblighter's camp to unlock the secret level, "Limbs, Heads, and Smoking Craters".
When getting past castles, have your troops hug the wall until they are propelled to climb stairs and take down the enemy archers themselves. If your troops hug the wall on the way in, the defenders will destroy their own structure.
Play level 3, "Down A Broken Path", ando to the top left portion of the map to find a small lake. On one of the rocks in the middle of the lake are three frogs. Send a dwarf up to blow them to pieces with a Molotov cocktail. Walk south to Tallow and complete the level. This will unlock the "A Long Awaited Drinking Party" secret level.