Defense game help

Ok, im making a defense game and i have the basic fort and gun modeled,but what i need to do know is to figure out how to make the grid fomation that is in normal defense games. i want to be able to place guns on one square and another on the next.

then after that i will have to do research on making the fight and and stuff:P

So to sum it up,I need a tut or a good read to show me how to do this:

           <b>Pls i really need to know this,i cant work on much more with out it!!</b>