The system of fortifications such as castles and fortresses emerged in Western Europe during the division of the Carolington Empire. Because of this system, the significance of decisive battles was greatly hampered since, even if an invader massacred large portions of a defending force, they would have to siege any fortifications that would not surrender. The local populace, if they can flee to the castle in time and are let in, are granted safety from the danger of an occupying force. The best example of this in practice I can think off the top of my head is the Hundred Years' War, where while the English won victories like Agincort, they were unable to subdue the French due to needing siege everything within a day or two's march to secure the countryside.
Moving on to the Byzantines, by time the First Crusades comes around under Alexios I Komenos, this fortification system has been in place for over a century. During the Crusade, Alexios had a large amount of contact with Western European rulers. During this contact, he learnes of the fortification. After the Crusade is over like in OTL, the Komenos put a lot of funding into building castles and forts on the frontiers of the empire in Central Anatolia and the Balkans. The Empire then begins emphasizing anti-siege tactics and strategy in its military doctrine, replacing the emphasis of the Byzantine Army's manpower strategy that relies upon militia and foreign mercenaries.
With the fortifications in place, how does the Byzantine Empire develop? Would the Turks be able to penetrate Central, and then Eastern Anatolia? If the Angeloi still come to power, would the fortifications avert or curtail the rise of the Second Bulgarian Empire? If the Fourth Crusade still has the Sack of Constantinople, would these fortifications frustrate the formation of the Frankokratia? How would the Palaiologian restoration be affected, if it happens?
Moving on to the Byzantines, by time the First Crusades comes around under Alexios I Komenos, this fortification system has been in place for over a century. During the Crusade, Alexios had a large amount of contact with Western European rulers. During this contact, he learnes of the fortification. After the Crusade is over like in OTL, the Komenos put a lot of funding into building castles and forts on the frontiers of the empire in Central Anatolia and the Balkans. The Empire then begins emphasizing anti-siege tactics and strategy in its military doctrine, replacing the emphasis of the Byzantine Army's manpower strategy that relies upon militia and foreign mercenaries.
With the fortifications in place, how does the Byzantine Empire develop? Would the Turks be able to penetrate Central, and then Eastern Anatolia? If the Angeloi still come to power, would the fortifications avert or curtail the rise of the Second Bulgarian Empire? If the Fourth Crusade still has the Sack of Constantinople, would these fortifications frustrate the formation of the Frankokratia? How would the Palaiologian restoration be affected, if it happens?