"Philadelphia was located in an area that was rich in agricultural crops, especially grapes, and the population enjoyed a certain amount of prosperity...an area subject to great earthquakes and had been destroyed several times..."
(Prophecy Knowledge Handbook, John Walvoord)
"Philadelphia...was destroyed in AD 17 by the same earthquake that toppled Sardis. Tiberius Caesar, the great builder of cities, reestablished it.
Philadelphia was at the center of a great vineyard district and had a thriving business in wine. Because of this, Bacchus, the god of wine, had many devotees there. Quite naturally, drunkenness was a chronic social problem in the district."
(There's A New World Coming, Hal Lindsey)
"Philadelphia was a city of Lydia, 28 miles from Sardis, in the valley of the Cogamis, a tributary of the Hermus, and conveniently situated for receiving the trade between the great central plateau of Asia Minor and Smyrna. The district known as Katakekaumene ('Burnt Region'), because of its volcanic character, rises immediately to the NE of Philadelphia, and this was a great vine-producing region.
"It was liable to serious earthquakes, but remained an important centre of the Roman province of Asia, receiving the name of Neo-Caesarea from Tiberius, and, later on, the honour of the Neocorate (i.e. the wardenship of the temple for Emperor-worship)."
(Hasting's Dictionary of the Bible)
Philadelphia was called "city of the open door."
"Janus, whose key the Pope bears, was the god of doors and hinges, and was called Patulcius, and Clusius 'the opener and the shutter.' This had a blasphemous meaning, for he waws worshipped at Rome as the grand mediator. Whatever important business was in hand, whatever deity was to be invoked, an invocation first of all must be addressed to Janus, who was recognised as the 'god of gods,' in whose mysterious divinity the characters of father and son were combined, and without that no prayer could be heard--the 'door of heaven' could not be opened. It was this same god whose worship prevailed so exceedingly in Asia Minor at the time when our Lord sent, by his servant John, the seven Apocalyptic messages to the churches established in that region. And, therefore, in one of these messages we find Him tacitly rebuking the profane ascription of His own peculiar dignity to that divinity, and asserting His exclusive claim to the prerogative usually attributed to His rival. Thus, Revelation 3:7: 'And to the angel of the church in Philadelphia write: These things saith he that is holy, he that is true, he that hath the key of David, he that openeth, and no man shutteth; and shutteth, and no man openeth.' Now, to this Janus, as Mediator, worshipped in Asia Minor, and equally, from very early times, in Rome, belonged the government of the world; and, 'all power in heaven, in earth, and the sea,' according to Pagan ideas, was vested in him. In this character he was said to have the 'power of turning the hinge'--of opening the doors of heaven, or of opening or shutting the gates of peace or war upon earth."
(The Two Babylons, Alexander Hislop)
See the online book The Letters to the Seven Churches of Asia by W. M. Ramsay.