Does someone know how to fix this?

Hi I would like to create a game in which there are towers made of several cubes and you can destroy these towers by throwing spheres to them. When I added Simulate physics on each cube, when I press play the building doesn’t stay in place: cubes on the top start to fall and the tower is destroyed. I tried to adjust the linear damping and it stopped but when I throw spheres to the tower it doesn’t destroy it as strong as I’d like. Moreover, when I turn off gravity it does the effect I would like but the cubes don’t fall on the ground so does someone know how to fix this?