I equipped the flame ladders next to the Doom Chest and still needed $8 worth of repair despite none of the enemies reaching the Doom Chest. Perhaps those multi-colored skeletons deal damage when they die on the ladders. I'm not as certain, anymore.
CAUTION: Explosive ladders can damage the Doom Chest if detonated next to it. After several tests, I found that each explosive ladder does $8 worth of damage to the Doom Chest. When both exploded, $16 of repairs were needed when 0 enemies reached the Doom Chest. Forcing enemies to run into the lava is fun, but it may not be worth losing the $1,000 awarded in the Flawless Doom Chest bonus. Tread carefully, fellow Trap Masters.
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