Logically speaking, a siege is an extremely arduous task that requires a significant amount of time.
However, for Kriolleiv, he only had four days to spare!
Yes, the Steel Giant would arrive at the Bonn Fortress in four days. By then, with the overwhelming firepower displayed by the Steel Giant, it would be absolutely impossible to breach the Bonn Fortress. Therefore, these four days of window were the only opportunity!
Kriolleiv surprisingly didn't have any decent plan for this. Fortunately, even though the Bonn Fortress has the position of the Tiger Cage Pass, it lacks its strength. It is merely an ordinary military transfer fortress of Herman. Thus, the original defense facilities are not sufficient, and the city wall is neither high nor robust.
In simple terms, Bonn Fortress is more like an enhanced version of a mountain stronghold. However, it's still inferior compared to border fortresses. What Kriolleiv originally saw as the biggest flaw was now his only advantage.