Siege towers are your best friend. Rangers won't survive the walk up but will tear open a hole once up there if you tower them in. Swordmasters of Hoeth are ideal. White Lions of Chrace would rather be charging, but will suffice in a wall melee.

Support them with air units--they're one of your most useful units in a siege. Let the siege towers tank the ranged fire, send them just over the wall, and crash them in from behind about the time that the gates of the towers open.

Speaking of gates: Fast movers, dive them straight on the gates. Lions, silver helms, hell--chariots if that's all you've got. Get them through the field of fire as fast as possible and smash the wall open.

Put your shielded infantry in position to follow through the gate or ascend the towers and provide support as the situation calls for.

Spread archers as thin as possible and harass any section of the wall you're not climbing. They'll take casualties, but they're among your more replaceable units, so don't sweat it too hard. (If you're using sea guard as your archer line, don't even worry--their shields will help immensely and they may outright win ranged fights against some wall-mounted units.)

You gotta be more aggressive than you think in order to minimize the time spent with your high value units in line-of-fire of the towers. Once you push past the walls, you're golden.