Merge the windows on two, three, four, or even up to a nine story building. Each numbered window block can be merged with another window of the same level to create a window of the next level. New windows drop from the outside columns and you can move them anywhere on the sides of the building while you're waiting to merge them. Complete the Windows on the building before the Windows reach the top of the screen.
The hardest levels have a building the full height of the playing area so you cannot share windows from either side of the building. It's like playing two fast paced games at the same time.