In the mid -17th century the Portuguese had a trading outpost in the area at Sutanuti, followed by the Dutch, who constructed a diversion canal at the bank of the Hugli River, near the present Central Business District. The old Fort William was built to protect the English post in 1696. The city became famous in 1756, in England particularly, when Siraj-ud-Dawlah, a Bengal ruler, captured the fort and, according to British historians, stifled to death 43 British residents in a small guardroom called the Black Hole of Kolkata. The city was recaptured by the British under Robert Clive in 1757. The English initially built an intricate transport network through the Hugli - Ganges water system, but it was the railroads, introduced in the 1850s, that successfully established connections with the hinterland and the rest of India. The city eventually had the largest concentration of trading establishments in India, and a Western-style business district evolved by the end of the 19th century. The colonial city maintained a strict division between the crowded and ill-planned native quarters to the east and north of the Central Business District, and the spacious and well-planned quarters where the Europeans lived in the south and southeastern parts of the old city. After independence, the former European quarters were either turned into residences of the Indian rich or, as in the Park Street area, into commercial areas.