For the first round that moves in from the Levant (Oghuz/Crossbows/Trebuchets), I simply fortified Baghdad with Crossbows and picked the stacks off with Horse Archers/Cataphracts (I'm assuming this round is meant for the Arabs, as it spawns around 1070, and would be incredibly doable for them with Camel Archer spam alone, although defending Persia is more dubious). This round of conquerors was much more enjoyable than the usual manner of conquerors: the stacks spawned outside of Baghdad's BFC (kind of in Western Iran), and then moved into range over two turns. I found this to be much more fair than the usual system of just materializing stacks right next to your cities, which is exactly what the old Seljuk conquerors (and the second round of Turkics) do. The Turks sued for peace after dismantling 1.5 of these stacks, and then re-DOW'ed with the second round.
The second round at 1120 AD (Lancers/Crossbows/Trebuchets, which would have taken Anatolia the first time because I wasn't expecting it); reloading allowed me to reposition the garrisons from the Levant/Baghdad to Anatolia so that the AI would bombard the city defences down first (also didn't completely wipe the first round stacks this time, opting only to kill most of the combat units and leaving weakened Turkic Trebuchets to poke Baghdad slowly). From there, it was just picking off the stacks with Cataphracts. Having one of the stacks in the second round spawn west of Ikonion is kind of annoying, as they spawn on a hill which makes eliminating them a bit tough (the old Seljuk stacks spawned 1N of Ikonion, and 1SW of Trebizond, a plains and hill tile respectively; new Turkic 1120 spawns are 1W of Ikonion, and 1SW of Trebizond, both on hills). Turks sent in another round of units (leftovers from Persia I'm guessing) against Baghdad (~8 units) and after those died, they would have sued for peace at ~1240 AD.
Ottomans spawned as normal, albeit the Turks did not conquer a single city west of (and inclusive of) Baghdad. From what I understand of the conditions that will be implemented, if the conditional Ottoman spawn was implemented, for a Byzantine player to prevent an Ottoman spawn, they need to prevent any Turkic conquests west of Baghdad, which means defeating two rounds of conquerors. That is a lot of units, but it is doable. However, I'll add the following caveats:
- I cheesed the Arabs. The most important ramification of this is keeping the two Levant cities: Damascus is a reasonably strong production city that usually keeps decent infrastructure, and probably contributed about 6 - 8 units in total against the two stacks. Jerusalem provides 40+ gold worth of income per turn to sustain the economy. The two Egyptian cities come online a bit later, and are mainly there for commerce. I workshopped all floodplains, in both Egypt and Mesopotamia so that the cities would have production capacity. Cairo produced about 4 units. Without these units, I am not certain whether Byzantium can defend all of the necessary cities
- As a consequence of the cheesed Arabs, they put up no opposition to the Turks. This is kind of hard to predict, but seeing as a human Byzantium who wants to prevent Ottoman spawn needs to secure Baghdad, they will be warring the Arabs at some point anyways and wiping out most of their combat capacity (I DOW'ed at ~970 AD, captured Baghdad and menaced Mecca, and Arabs capitulated).
- Cheesed Arabs barely expand into Persia: when I capitulated them, all they had was the northeastern Persian city, Baghdad, Mecca, the Yemeni city, and a city NW of where Riyadh would be.
- I stopped producing units and switched over to infrastructure right after the first round spawned, because I didn't expect the second stack to spawn in 1120 AD. I ultimately WB'ed in 3 Cataphracts, but those would easily be producible out of the cities I had - Ikonion, Trebizond, Damascus, Baghdad, and Cairo all had capacity to whip out a total of 8 Cataphracts over 4 turns if needed, and each produced them in 7 - 12 turns. Athens, Constantinople, and Belgrade could also have produced more, as well as Naples and Alexandria.
- I also WB'ed to rig the Great People spawns out of Constantinople, because I was too lazy to S/L scum it.
- Mongol conquerors (following defeat of the second round of Turkic conquerors) spawned three very substantial stack (4 Keshiks, 4 Horse Archers, 2 Trebuchets each) - 1 next to Ikonion, 1 next to Trebizond, and 1 floating in northern Iraq. These are both larger than the older stacks Byzantium faced, which off memory tended to be 2/2/2, and also spawned right inside my core and not at the periphery. I would have only had enough units to defeat one stack (the one next to Baghdad, since that's where my units were), and would have lost Anatolia to the Mongols as a result. First time in all of my Byzantium games where the conquerors have spawned in such a way; usually, they spawn in between the Levant and Mesopotamia, in Egypt, or east of Mesopotamia. Only met Mongols due to making peace with the Turks and taking a city in the peace deal; reloaded and made it to the Ottoman spawn without seeing Mongols.
- As I noted above, it is it actually irrelevant how much tech (i.e. Commerce) Byzantium has: Crossbows, Horse Archers, and Cataphracts are all that are needed to beat the conquerors. It is Production that is the limit, and I could certainly have produced more units than I did here (even if I had accepted Arab flip), as I built quite a bit of infrastructure in the core cities, and also spent some resources reconquering Rome and Tunisia, and capitulating Arabia. Of the techs realistically reachable at Regent, only Commune and Longbows might have helped out.
