You can use this function to get the unique tile "id" for
all tiles in a room, regardless of their depth. The function
will return a 1D array, with each entry being the ID value for a
var tiles = tile_get_ids();
for (var i = 0; i < array_length_1d(tiles); i++;)
The above code will get the ID of all tiles in a room and populate the array "tiles" with them, before looping through the array and setting the alpha of each tile to a new value.