According to Greek researchers The Turks renamed Constantinople to Istanbul because of a misunderstanding.
After capturing Istanbul, the Greeks who were going to Istanbul were asked by guards the question:
-"Where are you going? (hoping to learn the name of the city they captured)"
The Greeks answered :
-"Eis ten polin," which meant "In the city (you dope-heads where do you think I'm going to)"
The Turks after hearing this answer thought the city they captured was named "Eist ten Polin" and mispronounced it as Istanbul thus changing the cities name.
Fortunately, there are other explanations.
According to Kazım Mirşan the old Turkic name of Istanbul was ASTAN-BOLIQ
-Mesudi who lived during 10. century wrote that this city was called ASTAN-BULEN by its inhabitants in his book “Efembih Vellişref”
-Batuta who lived during 14. century wrote the city was called İZTANBUL by its inhabitants
-Vartan who lived during 14. century wrote the city was called ESDAMPOL by its inhabitants.
-The city was walled İstanbul during the reign of Second Murat. http://en.wikipedia.org/wiki/Murad_II
Istanbul might have been initially settled and created by Proto-Turks.
According to Kazım Mirşan the name is Turkic;
*İSTAN: In ProtoTurkic means "connected to the "Upper-World (which is the Tengri's floor;empyrean) - "being connected to the heaven":''Tanrı katına AS/ılı olan"
The root is "AS/qan" it changed over time AS/tan, AS/pan , and today it's AS/üman.
*There was a SkyGod named İSTANU among Hittites. this is the heritage of ESTANU among Hattis, Hattians (E.Akurgal, Anadolu Kültür tarihi, Tübitak 1967)
*Also in Ottoman Empire another name for Istanbul was "Asitane" (ASTANA:ASİTANE)
*In East Turkestan (today occupied just like Tibet by China) there is city named Astana near Turfan:Turpan http://en.wikipedia.org/wiki/Turpan
*The Kurd rebel Şehy Sait's father is buried on the hill in Turkey which is named Astan Hill. (M.Toker, şeyh Sait İsyanı)
ASTAN was ASQAN in Proto-Turkic which meant "to be in heaven"
*City of Kastanbolu in Trace could also have a connection with kESTAN POL (ASTAN BOLIQ)
*Astana in Turkic means capital city. The capital city of Kazakhstan is indeed Astana. The word is Turkic and it reached everywhere in different forms where Turks settled in. The Kazakhs changed their capital name from BeşBalık (BÏŞ-BOLIQ) to Astana.
According to researches done in this area, Istanbul was first created by Vyzas in BC 657 and that this "VYZAS" word comes from "BYZANTIUM". This weird tale led serious researchers to find a more believable theory. Krethmer tryed so hard to find some truth in this and tried the words like Biezas, Buezas, Buesantis but couldn't reach any scientific result.
-According to myth, Vyzas the son of Poseidon (the God of seas, winds and earthquakes) created Istanbul.
Apparently Trojans were stupid enough to not colonize Istanbul according to this myth.
The first great Proto-Turkic confederation was named BİR-OY-BİL and their capital was named "AT-OĞI BOLIQ"
BOLIQ means site, ordered (as in packed like sardines or jam-packed) and abundance.
Today in Anatolian Turk language we use the word "BALIK" for fish and "BOLLUK" for abundance and "BALIK İSTİFİ" for "in an orderly fashion", "crowded" (I know it is strange but these are really the truth)
Nevertheless Balık is still used for some Turkic cities around the world and it was used much more in the far past.
*In Anatolia the name became Bolu (A city in Anatolia).
*There is city named "Can - BOL" near Aral Lake.
*Kastamonu probably is related with (K)ASTAN BOLU.
ASTANA in time changed into SİTAN, İSTAN (probably because of Persian influence) That's why there are many country names ending with STAN. In turkish we say Yunanistan (Greece), Bulgaristan (Bulgaria) Macaristan (Hungary), Ermenistan (Armenia), Frengistan (country of Franks), Türkistan, etc..
EDIT: fixed typos, these are just information. What I think is irrelevant.
After capturing Istanbul, the Greeks who were going to Istanbul were asked by guards the question:
-"Where are you going? (hoping to learn the name of the city they captured)"
The Greeks answered :
-"Eis ten polin," which meant "In the city (you dope-heads where do you think I'm going to)"
The Turks after hearing this answer thought the city they captured was named "Eist ten Polin" and mispronounced it as Istanbul thus changing the cities name.
Fortunately, there are other explanations.
According to Kazım Mirşan the old Turkic name of Istanbul was ASTAN-BOLIQ
-Mesudi who lived during 10. century wrote that this city was called ASTAN-BULEN by its inhabitants in his book “Efembih Vellişref”
-Batuta who lived during 14. century wrote the city was called İZTANBUL by its inhabitants
-Vartan who lived during 14. century wrote the city was called ESDAMPOL by its inhabitants.
-The city was walled İstanbul during the reign of Second Murat. http://en.wikipedia.org/wiki/Murad_II
Istanbul might have been initially settled and created by Proto-Turks.
According to Kazım Mirşan the name is Turkic;
*İSTAN: In ProtoTurkic means "connected to the "Upper-World (which is the Tengri's floor;empyrean) - "being connected to the heaven":''Tanrı katına AS/ılı olan"
The root is "AS/qan" it changed over time AS/tan, AS/pan , and today it's AS/üman.
*There was a SkyGod named İSTANU among Hittites. this is the heritage of ESTANU among Hattis, Hattians (E.Akurgal, Anadolu Kültür tarihi, Tübitak 1967)
*Also in Ottoman Empire another name for Istanbul was "Asitane" (ASTANA:ASİTANE)
*In East Turkestan (today occupied just like Tibet by China) there is city named Astana near Turfan:Turpan http://en.wikipedia.org/wiki/Turpan
*The Kurd rebel Şehy Sait's father is buried on the hill in Turkey which is named Astan Hill. (M.Toker, şeyh Sait İsyanı)
ASTAN was ASQAN in Proto-Turkic which meant "to be in heaven"
*City of Kastanbolu in Trace could also have a connection with kESTAN POL (ASTAN BOLIQ)
*Astana in Turkic means capital city. The capital city of Kazakhstan is indeed Astana. The word is Turkic and it reached everywhere in different forms where Turks settled in. The Kazakhs changed their capital name from BeşBalık (BÏŞ-BOLIQ) to Astana.
According to researches done in this area, Istanbul was first created by Vyzas in BC 657 and that this "VYZAS" word comes from "BYZANTIUM". This weird tale led serious researchers to find a more believable theory. Krethmer tryed so hard to find some truth in this and tried the words like Biezas, Buezas, Buesantis but couldn't reach any scientific result.
-According to myth, Vyzas the son of Poseidon (the God of seas, winds and earthquakes) created Istanbul.
Apparently Trojans were stupid enough to not colonize Istanbul according to this myth.
The first great Proto-Turkic confederation was named BİR-OY-BİL and their capital was named "AT-OĞI BOLIQ"
BOLIQ means site, ordered (as in packed like sardines or jam-packed) and abundance.
Today in Anatolian Turk language we use the word "BALIK" for fish and "BOLLUK" for abundance and "BALIK İSTİFİ" for "in an orderly fashion", "crowded" (I know it is strange but these are really the truth)
Nevertheless Balık is still used for some Turkic cities around the world and it was used much more in the far past.
*In Anatolia the name became Bolu (A city in Anatolia).
*There is city named "Can - BOL" near Aral Lake.
*Kastamonu probably is related with (K)ASTAN BOLU.
ASTANA in time changed into SİTAN, İSTAN (probably because of Persian influence) That's why there are many country names ending with STAN. In turkish we say Yunanistan (Greece), Bulgaristan (Bulgaria) Macaristan (Hungary), Ermenistan (Armenia), Frengistan (country of Franks), Türkistan, etc..
EDIT: fixed typos, these are just information. What I think is irrelevant.