The Gunpowder Units are a powerful group of units on Age of Empires II available in the Imperial Age. In order to create any of the non-Unique Unit Gunpowder Units, the technology "Chemistry" must have been researched from the University.
Despite being gunpowder units, Janissaries, Conquistadors, and Turtle Ships can be made in the Castle Age without the "Chemistry" technology.
The Gunpowder Units are a set of powerful Siege Weapons amongst other siege-like units. The full list is here: