Units can take out Petards.
Also, it costs so many petards to kill a castle (8 assuming none die on the way) that it’s usually not cost-effective for the Petards. If you’ve got units defending the castle (even your own Petards) this gets impossible.
Also, if you’ve never tried using Petards against Petards… They’re really good against each other. You’re liable to lose half your petard mass with one petard.
Yep, but it’s often shockingly effective. It’s definitely much preferrable to go imp and make trebs, but when it’s not an option for whatever reason, petards are the way to go
It’s only effective when it’s a surprise. In castle-age only it’s never going to be a surprise. Trebs aren’t an alternative. Any unit that can kill rams can kill Petards, and then some since archers aren’t negated.
I do agree that Castle Age siege is tough, but if it’s about taking out one single castle, petards will most often work. Send a raid to the opponent’s eco, use the distraction, go in with the petards. Is it guaranteed to work? Absolutely not, but there aren’t (or shouldn’t be) any strategies that 100% work without fail 
On the other hand, if someone builds like 2-3 castles into the same spot in Castle Age, that’s probably pretty much invulnerable to everything that’s possible in Castle Age. So there would have to be some kind of “can’t place castles too close together limitation” for a Castle Age ending mode of the game.