Both sides have been balanced as well as the current state of multiplayer will allow. In other words I've always won regardless of what spot I choose and playing back and forth with the same people. Defenders have an easier time, better micromanagement player should attack. There is a 1v1 version of the map which heavily favours the defender, this map can also be played 1v1 if preferred (still favours the defender). Experienced players can defend using isengard/mordor/goblins to help balance. Attacker should generally be isengard cause their power points prefer it...but do as you wish. Instructions ingame.
A balance was struck between adding units for tactical purposes (mumakil) and what does/doesn't break traditional cannon of helms deep. Attacker does not need to use a builder, its just there incase games go on for longer and u want the option. The maps are really setup for people to play them how they want. The underlying concept is the Attacker needs to attack though. That's why I forced attacker to use isengard units, so they wouldn't spend hours building armies of rohirrim or whatever else and fail miserably attacking cause they gave the defender too much time to buildup.