Bug, or feature that I just don't understand?

In my current Moorish campaign, I own Tripoli. As this is an extremely large province, I want to build watchtowers along the road between Tripoli and Egypt so I don't have to constantly patrol for rebels. I have had my general roaming around the desert for 10 turns trying, but there is nowhere in the province that allows him to build a watchtower.

The same thing is also happening to me in Toulouse.

Am I missing something?

When I was playing a previous campaign as the Eqyptians, I had no problem building watchtowers all along the Tripoli road.

Also, in this campaign, I own Arguinn(sp?). For the first 5 or so turns I owned it, I was unable to build watchtowers there. Then, all of a sudden and for no apparent reason, I was able to build them.

I also noticed this same problem in RTW occasionally, but was never able to figure it out there either...

What gives?