In 1976, the architect Paul Rudolph bought the 19th-century townhouse at 23 Beekman Place where he’d had an apartment since the early '60s. While keeping the existing building as residences, he constructed his now-landmarked, multi-level penthouse on top of it: a steel and cement work of art that is rigorous and spare in its lines yet dizzying in its scope and form. Inside, beams clad in reflective material support a light-filled space with few walls, delineated by platforms and catwalks and cantilevered, landscaped terraces with spectacular views out over FDR Drive and the East River. For Rudolph, it was a kind of creative laboratory — and it’s also not hard to imagine it as a site for glamorous, louche, late disco-era parties. But how about a family home? Enter designers John and Christine Gachot, of New York’s Gachot, known for the warm modernism they bring to their high-end interiors.