I use catapults on level 6's, zero losses. Just finnished testing to be sure, zero losses with all my old numbers on the same level npcs.

Also, a lower level HBR level makes the ballistas take less time to run into the rocks/logs, therefor getting themselves killed faster, and less hits on the walls before they get pwnt.
(apparently, I dont really know from experiance, as my 10 cities all have level 9 or ten academys and my HBR is at 10)

Game mechanic change- MYTH BUSTED.
Thread over.