I forgot about sapping, because usually sapping costs me more men than I'm willing to lose.
The overall goal as a Sassanid army attacking a city is to get your Clibs/Cataphracts inside the walls. Once in, about the only thing that can stop them is a RTW-style Phalanx, and they don't exist in BI. Sapping works, as do onagers or other artillery. With those, you will be unstoppable.
An alternative I've used in my last Sassanid campaign was to recruit as many merc of the Roman front-line soldiers as I can (I forget their name). These guys are as tough as the best Roman soldiers, and I'll use them for tough sieges. If they start losing the wall-fight, I'll send in my clibs as fire-support, since their armor lets them shrug off most of the fire from the walls (at least the regular stone walls).
If you're interested in what I call an "exploit", you can turn on "fire at will" mode for your siege towers. On large/epic wall sizes, they fire gatling-style ballistae (this is a bug I think), which can clear a section of wall of any defenders in a very short timeframe. Even the arrows shot by regular stone wall siege towers are pretty effective. I don't use this because it's obvious this is a bug because there's no way that a single siege tower would be able to fire 30+ ballistae shots per minute with their tech. If they did, they'd use these in defensive towers (built into the walls), and/or in the field, and nothing else in the game has quite this kind of firepower.
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