Aafter about 5 failed campaigns with them I decided to ask myself this question. I'm curious to see what the general consensus is.
My 5 failed campaigns involved me slowly, but safely, taking territory. Each time was me ignoring the Byzantines and focusing on the middle east. Iconium was basically under constant attack but I always managed to hold it.
My only successful, and current, campaign had me blitzkrieging literally everything between Sarkel, Iconium, Bagdahd, Jeddah, Alexandria, and Dongola. And I'm talking sacrificing everything in between.
Sacking every city, and almost exclusively using mercenaries via the Jihad I called on Jerusalem on turn 2, I took every damn city in the Middle East. Within 25 turns I had everything mentioned above, and watchtowers set up 100% (including Jedda and Sarkel).
By besieging, cancelling, and re besieging Jerusalem I had about 10 generals (who lost entire armies through desertion) running around setting up watchtowers and annihilating everything minus Egypt. The second I took Jerusalem I had every Egyptian city taken by the next turn.
Now by this point I'm broke, broken, and extremely vulnerable. Until I realized a solid combination.
Every Turk city, with a Merchant Guild of any level, can have Saracen Militia and Siphis. Turn 68, one turn after the Mongols showed up, I'd completely destroyed the mongols first wave by Baghdad. Same thing with the second and third wave.
I kept Mosul and Cesarea castles, which I'm not even sure was necessary. Granted I'm 30 turns away from the Timurids so we will see. Yerevan I converted to a castle. Everything else I immediately switched to a city and turtled for 40 turns. Nearly destroyed my economy keeping 3 full stacks of balanced armies snd a single full stack of Siphis for nothing but clean up.
Post Mongol invasion I've made everything except Bran (as it was very developed), Ragusa (for my current Italian invasion), Zagreb (converted to castle for Italy invasion also), and Magdeburg (for Denmark/England invasion) into cities.
Now I'm playing on medium/medium to be clear. Are the Turks better off as a Milan-style faction? Because between City watched and Merchant Guilds across the board I'm having a hard time justifying castle units with them.