This story is Romantic in that it focuses, to a great degree, on human imagination and emotion. The descriptions of the masquerade's location, the seven rooms running from east to west, each decorated in a particular color scheme, are highly imaginative. Prince Prospero is, clearly, creative and artistic if not courageous or wise. Moreover, the masquers themselves are inventive to the point of being grotesque: they no longer seem human. "There were much glare and glitter and piquancy and phantasm [...]. there were arabesque figures with unsuited limbs and appointments. There were delirious fancies such as the madman fashions." The revelers are dressed in odd, frightful, bizarre, and even terrifying ways as if to please the prince and match the imagination he has displayed in decorating the rooms. The narrator even describes them as "dreams" more than once.
Further, the story likewise emphasizes the terrible emotions felt by the revelers when they hear the ebony clock in the last room, the room of black and red, strike. This room is "ghastly in the extreme," and the masquers avoid it almost completely. When the clock chimes, the prince's guests experience "disconcert and tremulousness" and seem to meditate on something that makes them "[grow] pale." All the gaiety ceases most especially when the clock strikes midnight (symbolic of death). Given that the fatal disease called the Red Death is raging outside, and the clock is black (symbolic of death) and sits in a black and red room (symbolic of death and blood), the chimes likely remind them of their own eventual and unavoidable deaths. They try to avoid death, just as they avoid the room where the clock sits, but it is impossible to escape death forever.
All of this focus on both human imagination and emotion is quite Romantic.