On level 74, right before you approach the citadel, you partner will tell you that it is, indeed, a trap.
Go into the woods to avoid the archers, and when the ninjas jump down, cast shoopusmahwhoopus and fire your lazor. Then rush up to the castle, screaming the code phrase "Leeroy, you are stupid as hell" and use the little boy as a battering ram to break down the castle walls. Then use 'Ultimate Death Mega Blast' and you should have Chuck Norris, Mr. T and Billie Mays now fighting on your team. The enemy knights will then all pull out shotguns and blast their own brains out. Then you can proceed to the inner sanctum and retrieve you pants.
Bloodharp has a point, but that's just how people picture them usually.
And for anyone who doesn't get the joke behind the last achievement, look up 'I'm on a boat' on YouTube.
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