CLASSIC EASTSIDE LIVING. Newcastle is a small, quiet Eastside city nestled on the side of Cougar Mountain. Located conveniently between Bellevue, Renton and Issaquah, the 8,375 residents of Newcastle enjoy a peaceful environment with a vast array of outdoor opportunities. With easy access to I-405 and I-90 at Factoria via Coal Creek Pkwy, Newcastle is an ideal location no matter where your destination may take you.