The ship-line of Mercenary Warships offers the Trebuchets among warships: long ranged artillery equipped ships with a great bonus against buildings and most land units. These units are available in the 15th century, but first require Alchemy and the Mercenaries technology to be buildable.
The Mercenary Warship Line consists of the Privateer and the Likedeler, pirate mercenaries.
For some civs, the names are changed to the Carrack and the Caravel, great sailing ships.