Ah... still a ship the size of a cruiser could still get underneath and tear up the underside. You may want to look at how EVE Online does there ships. Each ships has a full 360 fire arc around the ship but can only hold so many weapons (Gotta watch your power consumption and CPU usage). that's how things are done in my Skies of Fire universe.
Seismic's Signature
"To make an apple pie from scratch... You must first create the universe." -Carl Sagan
In most situations a dreadnought like this would be in a fleet of other vessels. Although it's slow it can turn, all it has to do is rotate as a ship tries to go under it to keep the turret cannons on it.
But whatever you do, don't fly in front of those fixed mass drivers.
Also, the torpedoes are homing so it could still hit ships underneath it with those torpedoes.