Each dungeon is made up of random layout of maps. The maps are all the same, but the order in which they appear is different. For example, one time you might see 1F, 3F, 4F, 2F, 5F, and another time it might be 2F, 5F, 4F, 1F, 3F. Each 1F is the same, it just appears in a different sequence. Each dungeon also has four bosses at the end, but only one can be fought at a time. This means that if you want to defeat every boss, it is necessary to complete each of the optional dungeons four times.