I have not watch the movie before, but I can say that by
100B.C., the Roman Empire in Europe has already began to build
roads and siege weapons like Onagers and
Ballistas. Besieging and capturing barbarian
settlements which has only wooden walls is really easy for the
Romans at that time.
If the defender has epic stone walls surrounding the city, there
are two recommended ways to overcome this obstacle, use either
siege towers or sappers.
For siege towers, your troops need to push the tower towards the
wall, once it reach the wall. Your troops can climb up the tower
and then jump down on defenders defending the walls.
For sappers, you can dig an underground tunnel towards the wall,
once the foundations of the wall is weaken, the wall will
collapse.
Arrows are ineffective against siege towers and totally useless
against underground sappers. 
