What stops flank attack?

[obsolete]

What were all my knights doing during this flank attack, sitting back and watching all my trebs be destroyed?

And why exactly... would you (ahem)... stack a bunch of knights with a bunch of trebs?

 

[deanej]

Actually, the proper counter has already been given: Let the enemy bring his SoD into your territory and wipe it out there.

I don't consider that a "proper counter" so much as a "workaround a bad mechanic". I would have changed seige this way:
-Eliminate the limit to number of units affected by collateral damage, making seige a counter to SODs as they were designed to be.
-Use the combat limit like Firaxis implemented
-Eliminate seige immunity to collateral damage

 

[bestbrian]

Use your espionage, and structure your stacks around what you'll be facing. Genghis has a mountain of Keshiks? LOTS of Formation Pikes. Better having him massacre himself against a border city? Let him do it. Capture his border city, and then pull out to lure in his stack, so you can destroy it with your CR promoted units? That works, also. You've got to be adaptable in your force compositon, and your tactics.