Let's see if I understood your issue: you want to have a grid but you don't want a Tilemap for whatever reason.
You can convert ANY set of coordinates (x,y) on the screen to a pair of integer coordinates (i,j) that correspond row and column (and viceversa) using some simple maths.
Basically, let's say you have a grid where each square has size
L. I want to build a building, and I click at the coordinates (x,y). We know that the x position of the
i-th tile is
x=i*L, so we can get its index as
i=floor(x/L). In the same way we can get the index of the y-position,
j=floor(y/L). Then, recompute the coordinates the tower has in the map. Try to draw in a paper and do the computations yourself, it is very easy!
I repeat the procedure to be clear:
1. Get the coordinates (x,y) you want to transform.
2. Compute the grid indices
3. Your tower will be located at
In this way you can get a grid from your script, without using any Tilemap. Ask if you have any additional question!