After Arnold invades the island, he plants some claymore mines inside the barracks of the feeble cannon-fodder foot soldiers. Claymore mines explode with hundreds of ball bearings; the claymore mine is an anti-personnel device with little "explosive" power. At any rate, Arnold plants the claymore mines in the barracks, all on the first floor. The barracks are flanked on either side by watchtowers. When Arnold detonates the mines, the barracks explode... as do the watch towers. It's not the framework leading to the watchtowers that explode, it is the rooms on top of the towers itself. If Arnold didn't plant anything in the towers, and the claymores are a low-explosive anti-personnel mine, how would the towers have suddenly erupted into flame and smoke? Absolutely a slip-up.