Delhi Sultanate
The Turkic and Afghan sultanate that ruled northern India for three centuries before the Mughals, and kept the Mongols out of it.
Before the Mughals there was Delhi, and for 320 years Delhi was ruled by soldiers who had arrived from the northwest as invaders and stayed as sovereigns. The Delhi Sultanate was founded in 1206 by Qutb al-Din Aibak, a Turkic slave-general of the Afghan conqueror Muhammad of Ghor, and it ended in 1526 when Babur shot Ibrahim Lodi's army to pieces at Panipat. In between, five dynasties fought each other for the same throne while together they did two things that shaped the subcontinent permanently: they made Islam a governing power in India rather than a coastal trading presence, and they stopped the Mongols at the Indus.
Five dynasties, one throne
The sultanate is conventionally counted in five houses: the Mamluk or Slave dynasty (1206 to 1290), the Khalji (1290 to 1320), the Tughlaq (1320 to 1414), the Sayyid (1414 to 1451), and the Lodi (1451 to 1526). Succession was rarely peaceful and almost never hereditary in the smooth sense. The founding dynasty is called the Slave dynasty because several of its sultans began as military slaves, purchased boys raised into command, which tells you how the system worked: power went to whoever the army would follow. Iltutmish, Aibak's own slave and son-in-law, consolidated the state. His daughter Raziyya ruled in her own name from 1236 to 1240, the only woman ever to sit on the throne of Delhi, before the Turkish nobles killed her for behaving like a sultan.
The high point of ruthless competence was Alauddin Khalji, who reigned from 1296 to 1316. He fixed prices in the Delhi markets by decree, ran a war economy of standing troops paid in cash, banned alcohol and private assemblies of nobles, and sent his general Malik Kafur raiding as far south as Madurai. The low point of ambitious incompetence was Muhammad bin Tughlaq, who moved his capital and much of his population 700 miles south to Daulatabad and then moved it back, and who issued token coins in brass and copper that his subjects promptly forged at home until the currency collapsed.
The wall the Mongols did not cross
The sultanate's largest geopolitical achievement was defensive. In the same decades that Mongol armies erased the Khwarazmian empire, sacked Baghdad in 1258, and ended the Abbasid caliphate, they hit the Punjab repeatedly and were repeatedly thrown back. Alauddin Khalji beat off a run of invasions between 1298 and 1306, executing captured Mongols outside Delhi. India was the great civilization on the Mongols' route that they never conquered, and the reason was a militarized frontier state with cavalry, cash, and forts already in place.
That shield had a cost. The sultanate ran on constant extraction to pay for constant war, and when the shield finally failed it failed catastrophically: Timur crossed the Indus in 1398, stormed Delhi, and left the city a charnel house it took generations to recover from. The Sayyid and Lodi sultans who followed governed a rump.
What it left behind
The sultanate made Persian the language of government in India, a status it kept until the British replaced it with English in 1837. It brought paper, new irrigation, and a fusion architecture of arch and dome grafted onto Indian stone craft, of which the Qutb Minar in Delhi is the announcement. It gave Sufi orders, above all the Chishtis, room to become the most durable popular face of Islam in South Asia. And it created the political unit the Mughals inherited: a Delhi-centered state that claimed the Gangetic plain and pushed into the Deccan.
When Babur won at Panipat in April 1526, he was not founding something from nothing. He was capturing a going concern, with a capital, a revenue apparatus, a court language, and three centuries of precedent for how a Muslim dynasty rules a majority Hindu population. Almost every argument the Mughals later had with themselves, about tolerance, taxation of non-Muslims, and how much of India one throne can actually hold, had already been rehearsed in Delhi.