In Tiny Heroes on the iPhone, there is an achievement that you can get for repairing 200 defenses. However, I don't see any way to repair defenses.
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Apparently, it is an actual Defense that you use, not a general game mechanic that you need to use. It unlocks in the second campaign.
From a thread on Touch Arcade:
Yeah, we're going to reword the description of the "Repair-Man-Man-Man" Achievement to indicate that it's an actual Defense you use, and not a general gameplay mechanic that players haven't discovered.