Firstly, there were many times when the empire was not on the defensive. The reigns of Justinian, Heraclius, Basil I, Basil II and much of the Komnenan period were times of military dominance where the empire was strong and secure. Culturally, until the very end Byzantium and Constantinople were the centre of cultural and artistic life in the west. Even now we can feel the influence of Byzantium and Byzantine culture from Saint Mark's church in Venice to Russia to the Pyrenees all the way to Scandinavia. So culturally Byzantium was (almost) never on the defensive.

Personally I think that the turning point where the collapse of Byzantium became inevitable was in 1204, when the crusaders went and sacked Constantinople. If you have a look at this map you see that the Angeloi dynasty were not doing so badly for the empire in a territorial sense. We should note that the Angeloi were famed for their incompetence, losing control of the Dalmatian Coast to the Venetians, allowed the resurrection of the Bulgarian Empire and failed to stop various invasions by the Seljuk Sultanate of Rum, losing Cilicia forever. Despite mismanagement by the Angeloi Constantinople was still the richest city in the world by some considerable margin and the Empires rich financial resources meant that there was no reason why a determined and competent ruler could not have kept the empire strong. Then Enrico Dandolo appeared with the fourth crusade in tow and ruined everything.

Compare this rich, prosperous empire with the capital which was "the city of the worlds desire" to the empire after the collapse of the Latin Empire of Constantinople. Constantinople itself had been thoroughly plundered by the Venetians and the Franks and its wealth and relics had been pawned by the Latin empire to pay for mercenaries. Many of its richest provinces in the Pelleponese had been lost. Andronikos III tried and very nearly succeeded in reviving the fortunes of the flagging empire but the rise of the Ottomans put paid to that idea.

In conclusion, I think that Byzantium was not doomed to collapse, as no state is doomed to collapse. However in my opinion from 1204 on the empire was doomed to collapse. It is really quite remarkable that it managed to struggle on for as long as it did.

The sentiment that the Byzantines were in terminal decline for a thousand years has always struck me as an odd sentiment, like asking what a child could have done to avoid dying at 112. You'd think someone would look at a state that existed from antiquity and lasted until the cusp of the Renaissance and ask "What did they do right?"

Is there a tipping point that sealed its fate?

My favorite answer to this question is when the Ottomans took control of the Gallipoli peninsula in 1354, establishing their bridgehead in Europe. There was a devastating earthquake that levelled the Greek settlement and the Ottomans shrewdly crossed the straits and dug in. Using their bridgehead, they swiftly overran most of what remained of the Empire in Thrace and began to wipe out the other enfeebled Balkan powers.

Could it have changed small or large policies to strengthen its base at home?

The Byzantines loved civil wars and succession disputes. Sometimes this led to a change of leadership and a surge of vitality (the usual suspects like Heraklios, Alexios), but the Palaiologoi would sell each other out to the Turks for short term gains that gradually doomed the Empire. Look at the tragicomic reign of John V spent in usurpations and counterusurpations. His three successors all made extraordinary efforts to save the Empire, but by then there was nothing left by Constantinople, Thessalonica (until 1423), and the Morea. Only a massive international intervention would have worked.

Byzantium was as much doomed to collapse as any other state in human history was/is doomed to end eventually. Nothing, especially nothing manmade, lasts forever. In the popular consciousness Byzantium's history is often - if considered at all - seen as a story of steady decline. Indeed if you race through the centuries in which the Byzantine Empire existed you can watch it shrink down from the mediterranean wide empire of Justinian to what is almost a city state in the 15th century. Even in phases of growth like under the Middle Byzantine soldier emperors it could never fully recover all the territories lost in previous centuries.

But this view not only overlooks how utterly huge an achievement it is for any state to last for more than a millenium but also how great the external challenges were that the Empire faced in the course of that timespan. Time and time again it had to defend itself against the often huge empires of the east (Sasanians, the Arab Caliphat, Seljuqs, etc.), wave after wave of nomad coalitions from the northeast (Huns, Avars, Bulgars, Pechenegs, etc.) and also attacks from the Latin West (the Normans of Sicily, the 4th Crusade, Charles of Anjou, etc.). The impressive thing is that Byzantium survived almost all of those foes by a very wide margin. From the states Byzantium interacted with in the 6th century like the Frankish or Lombard kingdoms, the Avar Kaghanat or the Sasanian Shahdom not one still existed four centuries later. From the 7th to the 9th century Byzantium faced an opponent that dwarfed it in size and resources in the Arab Caliphat. It not only defended itself successfully but also watched its rival crumble into smaller states. The Ottoman Sultanate, the state that eventually conquered Byzantium for good, only arose in the last one and a half centuries of Byzantine history. Even shortly before the Byzantines had seen the rise and fall of a variety of rival states like the Second Bulgarian Empire or the Empire of the Serbs.

That the Byzantine Empire eventually succumbed to its enemies isn't really much of a surprise. That isn't to say that the Byzantines never did anything wrong in the defence of their Empire. The loss of Anatolia after the battle of Manzikert in 1071 is at least as much to blame on Byzantine infighting as on the Turkmen invasion. It was a deposed Byzantine prince that invited the knights of the 4th Crusade to help him regain the throne of his father and only when he couldn't deliver on his promises was Constantinople sacked. The Ottomans could seize their first stronghold on European soil because they were invited as mercenaries by one party in a Byzantine civil war.

There are three major setbacks that did did cost the Empire much of its territory and strength: in the 7th century the Arab Caliphat conquered the Levant, Egypt and Northern Africa, depriving Byzantium of its most valuable provinces. In the 11th century the defeat at Manzikert did cost the Empire Anatolia, which had been its core area ever since the Arab expansion. Only parts of it could be regained. The last major blow came in the form of the 4th Crusade's capture of Constantinople in 1204 and the subsequent conquest of much of the Empire by the crusaders. Even after that the Byzantines managed to reestablish themselves in Constantinople and much of the Balkans. But in the end the number of foes proved too much, especially when their advance was aided by the constant dynastic struggles that marked much of the last centuries of Byzantine history.

I'm not going to say that any one of those three setbacks was totaly unavoidable. But Byzantium's eventual fall probably was. Because even if the Empire would have survived longer, one has to ask oneself how it would have reacted to later challenges. How would they have handled Timur's invasion of Anatolia at the beginning of the 15th century or Safavid expansion in the 16th? What about another try by western forces to reestablish the Latin Empire? Would the expansion of Habsburg and Russia into the Balkans have proven dangerous? Would the challenges of Modernity and Nationalism have had the same devastating effect on Byzantium as on the Ottoman Empire? In the end, no single state lasts forever.

I take the view that after the loss of central Anatolia the Byzantine Empire became effectively the city of Constantinople. The walls of the city and the blessings its location allowed it to hang on for another four centuries. It was the introduction of Artillery in the 15th century, that obsoleted the walls and doomed the city and thus the Empire.

There are many events in Byzantine Empires long history that if things had gone another way, the Empire might have endured. If the Byzantine-Persian wars had been less costly, or just been avoided all together, then the Arab Caliphate could have been contained. If the 4th Crusade arrived was 2 weeks later and missed the tides, the sack may not have happened. If the Turks had been pushed out of Central Anatolia, there might have been no Ottomans to extinguish the empire.

Between each major contraction, there were decades that a strong and competent leader could have turned the situation around. Even after the sack, there were centuries to turn the situation around. However, once cannon became decisive, it undermined the physical security of the city that was the last cornerstone that the empire rested on.