Even by the standards of luxury condominiums on the Upper East Side, Casa 74, the 30-story tower at 255 East 74th Street is a strikingly luxurious development. Designed and marketed as an "ultra-luxury" residence, Casa 74 offers an ambitious combination of amenity-laden living and deluxe condominiums, with a price tag to match. In short, it looked like the sort of building that would have a hard time during the current economic downturn. But it hasn't turned out that way, and Casa 74 has unexpectedly bucked a down real estate market, and is reportedly now 90 percent sold.Casa 74's combination of luxury condominiums, copious amenities and glass-intensive looks isn't a new one among new construction condominium developments on the Upper East Side, but it has to mean something that, even with Manhattan real estate listings cutting prices left and right, Casa 74 is having little trouble moving its luxury condos. Score one for amenities and generous square-footage, as well as the enduring worth of a condo on the Upper East Side, and chalk it up as a defeat for, um, broad economic trends.