"In my opinion, we should go out and fight them frontally as we did in the past war." added Duke Crestflare.
The Nergal kingdom launched many attempts to break through the Windshire Fort in the past. Some of their attempts were successful, but later they got pushed back with the help of King Leon and other Duke/Duchess.
In all their successful attempts, the Heart Kingdom always decided to stay defensive on their wall. Since then, the Heart kingdom has always faced the Nergal kingdom's army frontally rather than hide behind their wall.
The Fortress wall may be tall, have a height advantage, and be equipped with hundreds the defensive runes. But, they are only helpful during the beginning of the siege. As the battle progresses, they will lose their wall advantage more.