Good points, Roxxas. I think the timing of Sin's emergence is important. Sin arose after Zanarkand was nearly destroyed by Bevelle. As far as Bevelle was concerned, the war was probably over - and then, when they thought they'd won and were feeling complacent, Sin came out of nowhere and started blowing stuff up. I'd guess none of their weapons were designed to fight a giant flying or swimming monster that's as large as a city - they'd be ground weapons, made for striking troops and cities. One of the most powerful weapons, Vegnagun, was deemed so dangerous after its creation that it was sealed under Bevelle and never used. I'd guess that, by the time Bevelle thought of unleashing its doomsday weapons, it was already too late and Sin's attacks had destroyed the government and military leadership, leaving everyone and everything in a panicked tangle.