Why the fuck is Arte joining the war, it'd be fucked up as is to be a tourist at a warzone, it'd be unthinkable if she gets captured.
On the other hand if I were to write the story after she said she can make a doll dance Van would have immediately asks if she can control other object like say a mithril stick. And turns out she's basically Magneto and paired with good eyesight, any enemy she can see will get a mithril needle ran through them before they can react.
So in this scenario, after demonstrating the city's defense to the king and now asked to go on the offense and bring his army. Van will just bring Arte and say she is his secret weapon, all the crap he created for the town's defense is to distract from Arte surgically killing them en mass with precision.
And also Arte is super into killing people and they're playing a game of time attack where she records how fast she can send the needle out into the commander and back.