Fortified Borders

Numbers 21:21-24 - Israel sent messengers to say to Sihon king of the Amorites:

"Let us pass through your country. We will not turn aside into any field or vineyard, or drink water from any well. We will travel along the king's highway until we have passed through your territory."

But Sihon would not let Israel pass through his territory. He mustered his entire army and marched out into the desert against Israel. When he reached Jahaz, he fought with Israel. Israel, however, put him to the sword and took over his land from the Arnon to the Jabbok, but only as far as the Ammonites, because their border was fortified.

You don't hear about fortified borders much back in Old Testament times. Fortified, walled cities, yes, but not so much with the borders. I wonder how the borders were fortified. Were there walls there too? Were there towers? Guard stations? Were the borders fortified around the entire region? If so, what happened when you conquered more land?

The fortifications must have been fairly significant to deter the Israelites from attempting an attack. Then again, that's not really the direction they were heading.

I'm curious. Could you please tell me what brought you to this page by mentioning it in a comment? I won't publish the comment, if you ask me not to.