Ruling with Turks

Turks are a very impressive civ especially in the late game. Their unique unit Janissary can take out Infantry and Cavalry with ease , but are vulnerable against spear men and archers. So try to avoid engaging civilizations like Britons , Chinese and the Mayans. What make this unit unique is there incredible damage , especially so when attacking as a bunch. A fleet of janissaries can take out a cavalry fleet easily if used correctly. Turks also have an additional +2 range for bombard cannons , bombard towers and cannon galleons. This is also very useful since you can take out any castle except the Teuton castle without getting hit , so you don't need to build Trebs.
One disadvantage of playing turks is that they are gold heavy civ so you need to protect your trade route by all means. Since your best unit is produced from the castle you need a lot of stone to build few castles. And if the enemy take outs few of your castles you'll be helpless since your cavalry or infantry is not that strong. So it is always better to be defensive in the early game since you can rule in the late game.
Best way to counter Turks is using archers. Try to rush him with some archers early and take out their gold and stone miners as quickly as possible so he want be able to build some castles quickly. Harassing the trade route is also a good way to slow down the turks , since they are heavily gold dependent. Another this you must avoid with Turks is try to attack a fleet of Janissaries with a small forces of Cavalry. I have seeing countless players trying this and coming back without killing a single janissary. Always attack with archers and you have a pretty good chance of coming out on top, Britons can totally own Turks because of their superior range.