Because airships need to raid gleam engines for fuel and fight rival pirates' airships the weapon systems would have the ability to point in all different directions. How would the rotation work for these weapons?
Mount them on the bottom of the ship.
From there, they can rotate 360° without potentially aiming at your own ship. This has been done with airplanes for some time in the form of a Ball Turret.
Actually during WW1 airships were actually found to be fairly immune to bullets. Although bullets would puncture the hull, it would not cause an explosion, and the hole produced was very small and air would only leak out slowly.
By far and large the biggest cause of airships being brought down was from weather. The large surface area of an airship made it susceptible to being blown around in even the lightest winds, and the drive to reduce weight but increase volume meant structure could only be light, so they would 'break apart' easily as different parts of the airship would be moving differently to other parts.
Their only advantage was in WW1 airplanes could not reach them high enough - but once that changed there was no militarily advantageous reason to use them (except for navy blimps in a reconnaissance role where there was no air support). Also incendiary ammunition began to be used, increasing their vulnerability as they can burn and fall.
If you were to have an 'airship battle', then we are presuming there are no airplanes, and that weather is not a problem, and they still had use militarily. Also I presume there are no missiles, as they would be quite useful against airships due to their vulnerability to explosive ordinance and their large size.
Their slow speed means therefore tactics used in an airship battle with the above assumptions would be kind of like a 3-dimensional ship-of-the-line battle. Defence then using cannons does make sense - although if targets are below you your range is almost infinite, but if they are above you there is much shorter range.
Therefore when entering battle you would want your airship to be higher than the other airship, as you would 'take command of the field' - much like being upwind in an age-of-sail battle. Upon engagement or detection, the first act of each captain would be to increase altitude as rapidly as possible, to gain advantage in range. The fragility of airships would mean if your ship is in range of the other, but not able to return fire, then there is a high likelihood you won't make it.
As this is so important, probably a turret on the top of your airship is not as valuable as increasing your height as fast as possible, so the weight penalty of a turret on top of your airship would better be spent on either reducing weight, or having more volume to increase buoyancy, or a vertical engine to push higher.
The conclusion therefore is a turret only on the underside of the craft would be needed, and only need to shoot at the ideal ballistic trajectory (depending on the ordinance).
Turrets have the ability to rotate 360 degrees around the horizon.
One battery can cover the upper half of the airspace, another battery can cover the lower half.
In this way the entire airspace is in sight of the firearms.
Depending on how fast and manoeuvrable your airships are, and the size and mass allowances, you will need more than just one or two ball mounted turrets.
These are not "ships of the line" and are capable of manoeuvre in all three dimensions, so this needs to be taken into account. To protect the entire envelope, you will need the following:
Nose guns, both to discourage head on attacks, and as your offensive armament. Tail guns, to discourage stern attacks, and provide coverage to areas obscured from sight by the tail assembly of the airship. Top and bottom turrets, as described, and waist guns to protect the flanks, especially when the enemy is close enough that the dorsal and ventral turrets cannot elevate or depress to engage.
Nose and tail guns can be in ball mounts or "stingers", and waist guns can be manned by gunners pointing them out of a hatch (as in WWII practice), or a glazed, streamlined cover can be placed over the hatch to protect the crewmen from the cold and slipstream.
B-24H Liberator showing off the nose, ventral and dorsal turrets, and one of the waist gunner positions. The tail guns are obscured by the tail structure in this angle
Radar guided tail "Stinger" on a B-58 Hustler
B-17c. The nose gun is in a ball mount in the glazed bombardier's station, and streamlined covers are over the waist positions. The ventral station is a "canoe" with one or two ball mounted machine guns on the bottom of the plane
20mm cannon in the tail gun mount of an HE-177
As you can see from the historical examples, there were many different ways to mount defensive armament on aircraft in WWII and beyond. As noted, small calibre guns are not particularly suitable for shooting down airships, so you might consider mounting some sort of rocket weapons with large warheads as the offensive weapons (this also allows the airships to attack ground and sea targets as well).
A Steampunk "Rocket torpedo" might look like this