I was annoyed by the stupid path-finding (not only that sometime soldiers cannot get into the tower, I have more than once experienced them jumping between the siege-tower´s bridge and the wall, all dead of course ) on large and epic stone walls too. Therefore I have disabled the option to upgrade the walls further beyond simple stone walls. Since that time my game become much more enjoyable and I would kindly recommend you to do the same.

Spoiler Alert, click show to read: 
How to do that? Open "export_descr_buildings.txt" file, and search for the entries marked in red colour and delete them

upgrades
{
large_stone_wall
}

and
upgrades
{
epic_stone_wall
}

The whole stone wall entry than should look like:
stone_wall requires factions { spain, britons, gauls, scythia, dacia, romans_brutii, egypt, romans_scipii, carthage, parthia, numidia, thrace, greek_cities, macedon, romans_julii, seleucid, saba, } and building_present_min_level government gov4
{
capability
{
wall_level 2
tower_level 1
gate_defences 1
gate_strength 1
}
construction 6
cost 3200
settlement_min large_town
upgrades
{
}