What were some of the strengths of the Ottoman Empire?

Any true power or strength that the Ottomans had were not really from themselves but from those they conquered and weapons trade between the Ottomans and the farther east.

The walls of Constantinople in 1453 were widely known to be the strongest and most fortified border in the world. There was a private seller from Western Europe (I can't remember his name) that sold Mehmed the Conqueror a mortar that could destroy the walls of Constantinople. This trader actually offered this mortar to Constantine VI, but they didn't need it as a walled city. So really, they were able to raid the city with this.

Also, the Byzantine Army actually placed large chains across the Bosporous Straits to stop any Ottoman ship. However, the Ottomans wheeled their ships onto land to destroy the walls as well.

So really, their strengths at this time only came from the innovations of others and "sucker punch" dishonorable moves.

In obtaining Constantinople, the Ottomans sacked many of the Greek Orthodox Churches, most notably Agia Sophia, which were and are known to be adorn with priceless amounts of gold on the icons, which are a dedication to God. (see hitler's attempt to take the Holy Mountain (Agios Oros) in Greece)

Byzantium contained riches in churches and in the economy unknown in its extent today. These riches are actually all over the world today.

But of course, the location of Constantinople was the center of civilization. It was between three continents and was the center of all land and maritime trade. Take Constantinople and you rule international trade at the time.This allowed for the infamous spice and luxury trade between the ottomans and Western Europe.

Now of course, they stole many of the Christian riches in Jerusalem and the Holy Lands.

All of this stolen gold allows for, of course, expansion of sea trade, hence bettering weapons ability and strength, and a better militaristic and strategic location in the world for self-sufficiency and power.

(As a side not, looking at a satellite view of Istanbul today and comparing that to paintings of Constantinople under Byzantium, there hasn't been much development going on. It seems to have deteriorated under muslim rule).