This is it.

Curtis muttered to himself as he came to stand before the gate that belonged to his pack; the gate that separated them from the other parts of civilization.

When he became Alpha, since his sister would probably turn down the offer, he would tear down the gate. Actually, he was sure that she would. Of course. How could he forget? Leo was the rightful heir to the Alpha throne of the White Sted pack, that means his sister would be a Luna. Yet that depends on Leo agreeing to take the ruling reins.

However, there is also the fact that they didn't know where he had gone-if...