Austin-based private investment firm Castle Hill Partners is in the midst of developing Trinity Falls, a 1,700-acre master-planned community in McKinney near North Central Expressway and U.S. 380. The development will bring about 4,200 homes to McKinney, which is one of the largest residential projects under construction in Dallas-Fort Worth. Trinity Falls will include 300 acres of open space along the east fork of the Trinity River. Castle Hill Partners wanted to build on the site because it…