How does Victarion’s longship have a cabin?

The “longships” of the Iron Fleet are actually small warships rather than proper longships: 

“He had set sail from the Shields with ninety-three, of the hundred that had once made up the Iron Fleet, a fleet belonging not to a single lord but to the Seastone Chair itself, captained and crewed by men from all the islands. Ships smaller than the great war dromonds of the green lands, aye, but thrice the size of any common longship, with deep hulls and savage rams, fit to meet the king’s own fleets in battle.“

Hence the cabin.