NEW LISTING -- FIELDSTON HISTORIC DISTRICT: Grand 6-Bd. Stone & Stucco, Castle-Like Mansion     This 6-bedroom, 5-full- and 2-half bath, stone and stucco mansion has a castle-like turret and decorative brick highlights.     Located at 404 West 246th Street, the house was built in 1929-30 and designed by Fieldston's preeminent architect Dwight James Baum.    Features include: a gracious center hall; living room with a beamed ceiling and wood-burning fireplace; spacious family room and library with floor-to-ceiling windows; circular formal dining room (in the turret); center-island eat-in kitchen (outfitted with granite countertops and double sinks, wall ovens and dishwashers); master bedroom suite (with gas fireplace, walk-through dressing room with abundant closets, master bath with whirlpool tub and separate glass-enclosed shower); fitness room; sauna; finished basement with recreation room and maid's quarters.     There is a wraparound patio and garden. The house has a Spanish tile roof, private driveway and an attached 2-car garage. The property totals approximately 0.43 acres.    Conveniently located for well-known private schools (Horace Mann, Fieldston and Riverdale Country School), the vast greenery of Van Cortlandt Park and transportation to Manhattan (including express buses and the No. 1 subway train).