building gives points (which then will be used as money). With the "money" you can then buy artillery, defence, special bricks etc
Just dont make it too much of a positive feedback loop. Make sure that its not a case of whoever is doing well just gets strogner and stronger. Have something in your design that lets someone who isn't doing as well to catch up.

This looks like the sort of game I could play with my kids. I'll be better than my 6 year old son at building a tower, if that makes me able to build better defenses then I'll get much too strong for him.