Using of "this" in the sentence. Can I use "a"?

In this Algerian town on a rocky outcrop sticking into the Mediterranean Sea, al Qaeda recruiters find work for idle hands.
This is first sentence in a report here:
I wonder why “this” is used here. The town isn’t mentioned before. Is “this” used because
there is only one town like that in Algeria where Al Qaeda recruits young men?
If I replace “this” by “a” does it mean that there are many town like that where Al Qaeda recruits young men but the author only mention only one of them.

“this Algerian town” may simply mean “the town described in the article that follows”. However, it’s not true to say that the town hasn’t already been mentioned: it’s mentioned in the byline “By Lamine Chikhi, DELLYS, Algeria” at the very top of the article. It is possible that “this” may refer to that mention. In any case, you can’t infer that this is the only town where such recruitment happens.

“In an Algerian town …” would also be fine, and would probably be more usual in that position. It also does not make any implication about how many similar towns there might be.

Thank you for the perfect answer!