In 1798 the civil engineer John Carlysle Stewart came to western Pennsylvania to resurvey the "donation lands" set aside for Revolutionary War veterans and found that the original survey had missed about 50 acres where the Shenango River meets Neshannock Creek, then part of Allegheny County. Early maps showed the Native town of Kuskusky at the spot. Stewart claimed the land himself and in April 1798 laid out the town of New Castle.
More about New Castle
New Castle became a borough in 1825, with about 300 people, and was at first part of Mercer County. On April 5, 1849, the governor signed an act creating Lawrence County, named for Navy Captain James Lawrence, with New Castle as its seat. It became a city in 1869 under its first mayor, Thomas B. Morgan, with about 6,000 people. Also in 1849, Old Order Amish families from Mifflin County settled to the north in New Wilmington, and with later arrivals from Holmes County, Ohio, the community became one of the largest Amish settlements in Pennsylvania.
Business grew with the Beaver and Erie Canal, which ran through the city and drew manufacturers with its transportation and access to raw materials, and railroads later replaced the canal. In the 1870s New Castle became a major hub of the Pittsburgh and Lake Erie Railroad. Its population rose from 11,600 in 1890 to 28,339 in 1900 and 38,280 in 1910, as immigrants, especially from Italy, came to work in the mills and nearby limestone quarries.
Sources
Wikipedia: New Castle, Pennsylvania