in narnia prince caspian why did they fallback at the first battle?

If you're talking about the raid on the Telmarine castle, then they had to fallback/retreat because the gate was coming down, which was the Narnians only way to escape, and the minatore wouldn't have been able to hold it up for long. So they had to escape before they got trapped in the castle courtyard and get slaughtered or killed out by the Telmarines.

If they had waited any longer to fight, the gate would have closed and they'd have been cornered and killed.