The organization of the last Byzantine military formation suggests that the last professional Greek soldiers at Constantine XI's disposal were the Emperor's personal bodyguard (100-400 according to some accounts?), likely to be nobles and well off merchants who could afford quality weaponry and armor, and possibly the crews that manned the 5 remaining galleys, these ships were described as unarmed, weaponry was possibly taken off the ships to reinforce the defense of the walls, the crews numbered around 1000 with foreign mercenaries mixed in.
The bulk of the Byzantine forces defending the city were comprised of 5000 levies, 1500 who composed of the original city-guard before the siege. The levies were described as cowards and incapable by the mercenaries. The reality is they were probably very lightly armed, with some crossbows, mainly relying on rocks and any other object they could drop off the walls, and had no prior military experience. Small artillery was available but somewhat unreliable as the recoil was dangerous and damaging to their own walls, although some were employed with positive effect on the hills behind the Theodosian wall to the counter-fire Turkish artillery that was firing into the harbor.
11 Genoese and Venetian galleys with 1000 crewmen - Galleymen were some of the best soldiers of the era, it was a physically demanding job and the combat was grueling and dangerous,well armed and armored, these mercenaries were the professional equivalent to modern era marines, while technically mercenaries they were largely unpaid and were more politically or religiously motivated to help the Greeks because of nearby Italian holdings in Greece and Crete.
So in all it's unlikely there were 15,000 defenders, the number is closer to 7-8000.
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loginGalley warfare at the Battle of Lapento
Also a project dedicated to the digital recreation of the monuments of old Constantinople -
http://www.byzantium1200.com/