I don't know how you managed to get Jannisary archers by the time the Mongols invaded but I just fought off both a Mongol and Timurid invasion as the Turks using this tactic. I concentrated on building succeeding wall levels and then built the Cannon towers after gunpowder discovery. You can just do this in time for the invasion. I then filled ctites most likely to be attacked with Saracen and Jannisary infantry. When the horde layed siege before they could build siege engines I went into battle mode. The Mongols appraoch in front of the walls close enough for the cannon towers to pund them and inflict heavy casulties without a sortie. This either caused them to with draw or made their siege assault in the next turn too weak to penetrate my wall of infantry. With the Mongols they destroyed most of their army just to sack Bahgdad and destroyed the remnants in front of Mosul. The Timurids destroyd themselves in front of Antioch. Only setback is the amount of money and time wasted to continue expansion while you fend off the hordes!