How could you get 500% defense bonus? Fully level'd St. Basil is 100%, Fully leveled Deal Castle is another 100%. The only other defensive boost is watchfire towers, and at 4% each, you would need 75 of them to make up the missing 300% to meet your magical number for valid defense, lol.
The simple logic seems to me should go like this: The developers clearly made the offensive boost capabilities of GB to be exactly half that of the defensive boost capabilities. For example, any fully leveled offense building gives only 50% attack boost, while its counterpart defensive GB yields 100%. That said, all things being equal, if someone invest their resources, FP's, etc into building full defensive capabilities (I think the most I've seen is still under 300%), then even with 100% offensive boost (fully level'd zeus and castel del monte), then the prevailing logic seems that 300% boost trumps 100% boost. Yet I crushed the 300% guy earlier with my only 100% bonus, just by blitzing him with tanks and I didn't even lose a single unit. Somewhere the math is not adding up correctly.