Place units/structures on a grid?

[gmgo]

I have a gridded map, which consists of 9x6 squares. I want to put each unit/structures (they are units, but since they won't be moving anyway they can easily be structures) on each square.

The idea is the following: an Instant build mechanism (though with cooldowns, like a mercenary from campaing) but when you build this unit/structure, I have to place it on a square from the grid. It will be placed on the CENTER of the square, and you can't placed another thing on that same square unless you remove the unit in it (or it dies).

Tips to do this properly? Or could you point me to the right direction =D ?

 

[Zhall]

Oh boy, i remember when i was working on Industrial Giant for warcraft 3... The only way to really force building to grids is by using a placement system based off the directional pad.

And in my case all resources had to be spawned on the grid, which would crashed the game if the array was allowed to go on for too long.