Here's an overview of the map, including call-outs that are helpful to know.
Cobblestone has seen many iterations over the years. Up until this current version, it's been heavily CT-sided. Since this isn't the case anymore, you will have to be a bit more creative if you want to score those rounds.
First, let's talk about the map itself. Most games tend to be centered around the B-site. There are several reasons for this. First of all it's easy to isolate defending players on B with smokes. Secondly the terrorists will generally use AK47 whereas you are stuck with M4A4 or M4A1-s. They can prefire the most common angles with a weapon that has the potential to kill you with a single bullet. You will have to react to them stepping out, and if you're fast enough you will still have to hit them at least twice.
Then you have the stairs between B-halls and mid. If they rotate from B to A they can simply drop down the stairs and be there in no time. It takes a lot longer to run up the stairs and rotate from A to B.
The lone soldier on A
Unless you notice that the terrorists have a tendency to attack A, I would recommend to play with only one defender on A. His job will be to support the B-players as much as he can as well as to push for information and call for backup if needed. A lot of the time he won't need his smoke, so he can throw it over to B and smoke off long. That way you'll have four smokes ready on B.
After that he should sit around corners at CT ramp, long A or stairs. He will listen for footsteps and check for flashes and smokes. If he thinks that they're coming to A, he should try to survive for as long as possible. If the B-players manage to take control over long B or other key areas around B, the A-guy can fall back to the bombsite and one of the players from B can rotate and support him.
If he can't see or hear anyone he can try to move in for the backstab through Z. If he does, timing is crucial. Wait for the terrorists to initiate their attack, or else there’s a good chance they will have at least one player waiting for the lurker coming in from behind.
Should problem arise over at B, your brave soldier from A shouldn’t hesitate. He can either go in through connector door and call for a flash from drop room or throw the flash himself. This flash will blind any terrorist tucked in the corner outside door, making it a lot easier to initiate the retake attempt.
On the next page: getting creative around B.