Unfortunately, our knowledge of what actually happened at Philadelphia is rather limited. Generally, historians agree on this general narrative^(1):

• In 1378, John V Palaiologos and Manuel II Palaiologos request aid from the Ottoman sultan, Murad I, to help them overthrow Andronikos IV Palaiologos.
• For helping John V and Manuel II, Murad I was to be granted, among other things, Philadelphia, the last remnant of the ailing Byzantium Empire in Anatolia.
• However, even through John V and Manuel II successfully usurped the throne from Andronikos IV, Philadelphia was not granted to the Ottoman sultan.
• In 1390, when Manuel II was campaigning alongside Murad I, they captured the city of Philadelphia and it was officially ceded to the Ottomans.

But, as we'll see, the last bullet point is a point of contention.

It should be noted here, that our source for this narrative is usually Laonikos Chalkokondyles, a Byzantine historian, who was born a full fifty years after these events. Naturally, it is hard to consider him a primary source on this event. And as he doesn't really cite any previous works, it is hard to see where his information is actually coming from.^(2)

Our other somewhat contemporary, surviving accounts suffer from similar issues as well. Doukas, a Byzantine historian was born around 1400; Aşıkpaşazade, an Ottoman historian, was born in 1400; and Neşri, another Ottoman historian, was born around 1450. However, these accounts notably differ from Chalkokondyles' in one key way: the city of Philadelphia surrender immediately in 1390, instead of being attacked and conquered.

This was rather common for the Ottomans to do and was offered to other Greek cities, such as Thessaloniki, during the Ottoman-Byzantine wars. So, while it is still possible that Philadelphia refused to surrender, this does significantly challenge the original narrative.

And, talking about completely changing the narrative, recently, some historians, such as Stephen W. Reinert, have even challenged the idea of the Byzantine emperors, John V and Manuel II, even being at Philadelphia during its fall to the Ottomans.^(3) This is due to Chalkokondyles' narrative being consistently chronologically messy and possibly faulty during this period.

Additionally, the third bullet point has also been explored. It is unclear whether John V Palaiologos and Manuel II Palaiologos simply refused to hand over Philadelphia in 1379 to the Ottoman sultan as part of the treaty or if the Philadelphians refused to become a part of the Ottoman realm.

When questioned by this, Norwich in his A Short History of Byzantium gives the equivalent of "I have no idea," when he says "Had John and Manuel retracted their promise of 1378? Or had the Philadelphians simply refused to submit to the sultan? We shall never know." And this is coming from a person that wrote three volumes on the history of the Byzantines!

Unfortunately, our primary sources are a bit thin to none on this event. Even Manuel II, who we have several surviving letters from during his campaign with the Ottoman sultan, doesn't remark on the event as far as I can tell. And the other surviving accounts, as mentioned before, are written decades after the fall of the city.

Conclusion

Overall, the fall of Philadelphia is just not a super well-understood event yet. We know the basic details, but even when we try to dig a little deeper beyond surface-level, we run into inconsistencies. For example: Was there even a battle at Philadelphia? Why wasn't the city integrated into the Ottoman realm in 1379? Were John V and Manuel II even at Philadelphia in 1390? This is definitely an area that needs a bit more research.

Bibliography

^(1) For example, see: Harris' The End of Byzantium, see page 8; Necipoğlu's Byzantium Between the Ottomans and the Latins, see page 129; Norwich's A Short History of Byzantium, see page 356.

^(2) Kaldellis' The Greek Sources of Laonikos Chalkokondyles’ Histories, see page 738.

^(3) Reinert's The Palaiologoi, Yıldırım Bayezid, and Constantinople: June 1389-March 1391.