Not only fire arrows, but fire rocks upgrade for siege.Hmm quite indeed.
The following might sound a little strange coming from me (since I'm the one lobbying for stronger buildings)
But can't we make wooden buildings (or buildings constructed mainly of wood) less resistant against fire arrows.
It would be more realistic.
A second upgrade for siege could also be made, and something like reinforced boulders or something, and do extra damage to stone buildings (whereas the flaming rocks would have less effect)
just a suggestion btw