Question: How did they get into the city at the beginning?

I'm watching this the first time based on the good reviews here.

At the beginning of the movie, where Steven has the "war machine", one person 'volunteers' and starts rolling it to the wall. It blows up before they get there which is amusing. Then they all rally to battle and enter the city.

This begs the question, if they were about to enter the city then why bring out the war machine then? There is no breaking of the wall or gate, the gate just seems to open for no reason.

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