GridExample

using UnityEngine;
using System.Collections;

public class TileDrawer : MonoBehaviour {
    public GameObject prefab;
    public Vector2 gridSize;

    // Use this for initialization
    void Start () {
        for(int x = 0; x < gridSize.x; x++) {
            for(int y = 0; y < gridSize.y; y++) {
                //GameObject go = (GameObject)Instantiate(prefab);
                GameObject go = new GameObject();
                go.transform.position = new Vector3(x,0,y);
                go.tag = "Grid";
                go.name = "Grid";
                BoxCollider col = go.AddComponent<BoxCollider>();
                col.isTrigger = true;
            }
        }
    }

    // Update is called once per frame
    void Update () {
        if(Input.GetMouseButton(0)) {
            CreateTile();
        }
    }

    void OnDrawGizmos() {
        for(int x = 0; x < gridSize.x; x++) {
            for(int y = 0; y < gridSize.y; y++) {
                for(int z = 0; z < 10; z++) {
                Gizmos.DrawWireCube(new Vector3(x,z,y), Vector3.one);
                }
            }
        }
    }

    void CreateTile() {
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        if(Physics.Raycast(ray, out hit)) {
            if(hit.collider.gameObject.tag == "Grid") {
                Vector3 pos = hit.collider.transform.position;
                GameObject go = (GameObject)Instantiate(prefab);
                go.transform.position = new Vector3(pos.x, pos.y, pos.z);
            }
        }
    }
}