Summon Berserkers

Any castle with multiple unblockable ways in isn't much of a castle! Still, it could be that the player's plan combined with the circumstances was really just that good. It happens sometimes, and should be okay. Players receiving their just desserts because of clever play is an important part of the game, and encourages clever play even more than horrible consequences do for foolish play.

As for the spell Summon Berserkers, I'd agree with other comments that it is a powerful spell, but not at the top of the 1st Level spell list. That spot is reserved for Sleep, which is the tactical nuke of low-level play. Summon Berserkers doesn't even get the number two spot, as that goes to Charm Person, at least until higher levels (at which point Sleep and Charm Person reverse position due to Sleep's limitations). That leaves Summon Berserkers in the number three spot. I base all this on many (many, many) years of play with ACKS' precursors, and a couple years with ACKS itself now. I also don't see any of these as remotely overpowered. Mages are glass cannons, and the point where these are really powerful is where the Mage has one to a handful of spells per day. Those had better be good!

Beyond considering the power of the spells, I'd look at the situation itself. Is it possible that judgements made rendered the spells more effective than they might have otherwise been? Did the berserkers achieve total surprise (how, when they're a bunch of howling madmen)? If not, did the beastmen get ranged attacks before the berserkers closed? Did they get to set versus charge, either inflicting more damage or slowing the berserker's approach for another round of ranged attacks? If there were more beastmen than berserkers, didn't the beastmen get multiple attacks (2 or 3 to 1)? Weren't they able to funnel the berserkers and surround them? Did the beastmen get to Cleave?

All in all, the berserkers are pretty fragile, with an average of 5 hp, and they tend to go down pretty spectacularly in a hail of Cleaves once something of any size hits them. They also aren't PCs or Henchmen, and I can't see any justification for the PCs to be able to control them (assuming the "do their adventuring for them" comment wasn't metaphorical). Give 'em instructions, and let 'em rip, then hope they do the right thing while they're berserk. Alex has mentioned "abusing" them at higher level as trap finders, but even there, so what? You've blown a 1st level spell for a max of four 1-in-3 chances of finding traps. Why not?

EDIT: Were you using a grid, out of curiosity?