When the basic hull is done it’s time to create gunports, gallery, railing and other detailing.
The gunports are simply cut in to the mesh below the Shell modifier. Add some new loop on the upper and lower side of the ports and adjust with the edge constraint button selected in the sub object panel. Delete the poly in the port and close the subobject. Switch on the Shell modifier and you got gunports. Now connect the vertices around them as shown on the picture.