Spawn a grid from code

your project so far

Assets/

Scenes/

Arena.unity

FloorBlocks (parent)

Scripts/

GridSpawner.cs

Prefabs/

P_Cube

you already havethis lesson adds

Placing 100 cubes by hand is a waste of an afternoon. Two nested loops and one Instantiate call do it in a frame.

animated diagram

x runs across, z runs into the screen, and one cube drops per cell.

The grid spawner

GridSpawner.cs

using UnityEngine;

public class GridSpawner : MonoBehaviour
{
    [SerializeField] private GameObject blockPrefab;
    [SerializeField] private int columns = 6;      // along X
    [SerializeField] private int rows = 6;
    [SerializeField] private float spacing = 1.5f; // metres between blocks
    [SerializeField] private Transform parent;

    void Start()
    {
        BuildGrid();
    }

    void BuildGrid()
    {
        // centre the grid on this object
        float offsetX = (columns - 1) * spacing * 0.5f;
        float offsetZ = (rows - 1) * spacing * 0.5f;

        for (int x = 0; x < columns; x++)
        {
            for (int z = 0; z < rows; z++)
            {
                Vector3 pos = new Vector3(
                    x * spacing - offsetX,
                    0.5f,                       // half the cube height
                    z * spacing - offsetZ);

                GameObject block = Instantiate(blockPrefab, pos, Quaternion.identity, parent);
                block.name = $"Block_{x}_{z}";  // readable in the Hierarchy
            }
        }
    }
}

1
Create an empty GameObject, name it Spawner, put it at 0, 0, 0.
2
Attach GridSpawner.cs to it and drag P_Cube into the Block Prefab slot.
3
Create another empty called Blocks and drag it into the Parent slot — all spawned cubes will nest under it.
4
Press Play. 36 cubes appear, centred on the floor.

Read the maths once

x * spacing turns the loop counter 0,1,2… into metres: 0, 1.5, 3, …
Subtracting offsetX shifts the whole grid left by half its width, so it sits centred instead of starting at the origin.
The fourth argument of Instantiate is the parent Transform. Passing it keeps the Hierarchy from turning into 36 loose rows.

Variations worth trying

inside the inner loop

// 1 — random colour per block
block.GetComponent<Renderer>().material.color =
    Random.ColorHSV(0f, 1f, 0.4f, 0.7f, 0.7f, 1f);

// 2 — random height, so it reads like a city
float h = Random.Range(0.5f, 4f);
block.transform.localScale = new Vector3(1f, h, 1f);
block.transform.position   = new Vector3(pos.x, h * 0.5f, pos.z);

// 3 — a wave, using the distance from the centre
float d = new Vector2(x - columns * 0.5f, z - rows * 0.5f).magnitude;
block.transform.position += Vector3.up * Mathf.Sin(d) * 0.6f;

Clearing the grid

C# script

public void ClearGrid()
{
    // backwards: still right if this ever becomes DestroyImmediate
    for (int i = parent.childCount - 1; i >= 0; i--)
        Destroy(parent.GetChild(i).gameObject);
}