The Turks during the AoE2 timeframe spanned both the Seljuk and Ottoman Empires. And while there was no gunpowder introduced until at least the 14th century, the Turks could have two unique units: the primary one from the Castle could be a unique Cavalry Archer, probably called Sipahi, whilst the Janissary would be available in the Imperial Age Archery Range, replacing the Hand Cannoneer.
As for the Castle Age unique tech itself, the name of the tech would be Akinci Raiders. The tech will let CA and hussars regenerate, maybe?