I have created a tower with many cubes. When I start the game, every cube just jumps up in the air. Can someone tell me how I can fix that problem.
Just guessing that all the cubes are rigid body or dynamic; you should try just stacking one or two, to make sure the tower holds before trying to make a building out of them. Try making sure that the collision margin is set to 0 (I think that’s correct), or giving the cubes a collision mesh that is simple and exact (i.e., each cube’s faces don’t overrun into the faces of the cubes next to them).
Ok, thanks shall try 
Also check the bounds are set to cube…
Enable Physics Visualization in the render options. That may help you.