Creating a round gridbased map with array

Hey all,

I build a small grid based map using an Array like this:

 for (int i = 0; i < rows; i++)
        {
            for (int i2 = 0; i2 < columns; i2++)
            {
                if ((i) % 2 != 0 && i2 % 2 != 0)
                {
                    //Spawn wall pieces on coordinates i/i2 (middle of playfield)
                    gameManager.SpawnObject(wall, new Vector2(i, i2));
                }

What I wanted to do next is create a map which is not just a square but more of a cirle so basically an Arena like the Colloseum.

How could I achieve this with an Array like above ?

If I understand you well, instead of having a square you want a circle.
You can assiociate your object array as pixel array then use common algorithme to create your pattern.
Here are some examples :
Fast algorithm for drawing filled circles?
Midpoint circle algorithm