From the 1890s through the 1920s, the United States became one of the wealthiest countries in the world, with scores of savvy industrialists making fortunes during the nation's rapid ascent. Corporate powerhouses in railroads, steel and consumer goods created a class of businessmen with incredible fortunes, as well as a desire to acquire the class and respect of their Old World counterparts. The result was a frenzy of extravagant building projects across the country. Adapting European architectural styles, especially those of the Renaissance, architects and designers created multi-million-dollar mansions and estates that became the modern castles of America's upper class, a building boom that hasn't been matched since. While we may be able to equal the Gilded Age when it comes to income inequality, it's doubtful even in the era of $100-million properties that today's moneyed set will create homes that rival the mansions of oligarchs past. Here's a look at a few of the more fantastic properties built during that period.