Joyous Tower is one of the level one dungeons in the game. When you enter, all of your pokemon's levels are downgraded to level one temporarily (all higher level moves are forgotten temporarily, too). When you leave, they return to normal. Similarly, when you enter you will lose all of your money, so put it all in the bank before entering. Before entering you must save the game, so make sure you have an Escape Orb.
This dungeon gets added to your list after saving the Latias duo and acquiring the Sky Blue Plains Friend Area.
There are 99 floors in this dungeon, but there is no need to explore them all. The only two reasons to come to this dungeon is to recruit the exclusive pokemon found here (see table, below) and get the Friend Bow from one of the Kecleon shops (it appears randomly in the shop inventory).