I don't know if this was the first film doing this, but it is funny that "Troy" made the same nonsense a few years later. What's the point for the defenders in fighting a battle outside the walls of the city, instead of sheltering behind them? That's exactly why walls were built, to begin with.
shareI'm no military strategist but I would think they were trying to keep the invading army as far away from the walls/doors as possible, if they enemies get inside it would be much harder to stop them from doing damage to the city.
sharePlenty of potential reasons. Keep in mind that the Scorpion King was defeated in that battle.
-Avoiding a drawn out siege. Hiding behind the walls could result in a very long affair. The city might not have the supplies to withstand a siege. If they dont, it might be more "cost effective" to face the invaders right from the start. Even if they did have the supplies to hide behind the walls, a siege is still a very disruptive scenario to that city.
-Arrogance and/or confidence of the defenders in their army. If the leaders are very arrogant, they would rather face the invaders in battle and not hide behind walls. If the defenders are just confident in their army's superiority, it might be an objective decision to use that strength.
-Stopping the invaders from moving towards other targets. If the defenders hide behind the walls, the invaders are either going to lay siege to the city or they are going to move on. Meeting them in battle right there might be the only way for them to stop the invaders from roaming the countryside burning fields and villages and sacking weaker cities/towns.