Alone
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If I understand correctly, Neela shouldn't have suffered any losses at all... Only what the sprites and angels manage to kill (and they won't kill much there!). Of course, if she's lucky and the ballista destroys the wall in 3 rounds, then a few more skeletons and horses will be killed, but they aren't a significant force against Neela's 24 defense at level 21!
There were attempts to destroy the ballista, but in the 4th round, the wall was successfully breached. Skeletons passed through, but the blocked arrows significantly thinned out the ranks, so there was no significant damage to Neela's troops.