楼 (=樓) (rou) indicates a multi-storied building and 閣 (kaku) a building located in a high place, such as a palace or a turret, with a building meeting both of these conditions generally called 楼閣 (rokaku).
「楼」（＝樓）とは重層の建物、「閣」とは御殿や櫓など高所の建造物を意味し、これらを総称して楼閣という。