Another  option could be undermining. It was used by the more technologically advanced civilizations too. Probably easier to do than ladders (just a guy with a pickaxe).

Like this? (next release will have it)  =============================================================================== == isles-r28  <- pack.zip    output: C:\x\mods\isles-r28, 12 files copied    mod.json: ...    ----------------------------------------------------------------------------    maps/scenarios/greek_isles.xml       renamed gaia/flora_tree_oak -> gaia/tree/oak  (3x, r27)    ----------------------------------------------------------------------------    maps/scenarios/delta.xml       renamed gaia/flora_tree_oak -> gaia/tree/oak  (3x, r27)    ----------------------------------------------------------------------------    2 maps: 2 changed, 0 needed nothing; ... =============================================================================== Result: no failures left - ...

Ladders are a common early siege method. It may be a good way to compensate for erm... less complex civs in case siege tower has a prominant role for others.

Aside from the Helepolis at Rhodes i'm not sure armored siege towers were that common. i think some factions could be able to uprade them to have artillery and to have rams (current model could be used for artillery siege tower since it has the firing port) Helepolis could be unique to Macedonians, a heavily armored tower with artillery (very expensive and you can only build one).  

As for their use, it does seem they were used more as archer platforms at first but starting in the Hellenistic period were ocasionally use to climb walls. Ladders would be perhaps more common for climbing (although probably more difficult to animate).