The key to a successful final confrontation is exhibiting just the right amount of hubris. The antagonist has to be so confident of their victory, the heroes are left with exploitable vulnerabilities. Its no fun if they suddenly pull an anachronistic hang-glider out of nowhere and sail over your walls, dropping newly-invented bombs on all your troops. No, a facade of invulnerability can only be broken by the discovery of an overlooked crack on its surface.
Ive planned seventeen, just in case they miss a few.
Likewise, the underling betrayal has to come at just the right moment. Too late, and there is no sense of justice. Too soon, and the heroes are picking their way across a battlefield full of corpses but empty of opposition. The snail changing her troops into farmers was something I did not foresee, and so it speaks well for her growing, barely controlled resentment. I think I can ensure she snaps at exactly the right time.
Hmm. Maybe the key to a successful final confrontation is project management. I shall update my resume accordingly.