Code

boardmanager:

using UnityEngine;
using System;
using System.Collections.Generic;       //Allows us to use Lists.
using Random = UnityEngine.Random;      //Tells Random to use the Unity Engine random number generator.


    public class BoardManager : MonoBehaviour
    {
        // Using Serializable allows us to embed a class with sub properties in the inspector.
        [Serializable]
        public class Count
        {
            public int minimum;             //Minimum value for our Count class.
            public int maximum;             //Maximum value for our Count class.


            //Assignment constructor.
            public Count (int min, int max)
            {
                minimum = min;
                maximum = max;
            }
        }


        public int columns = 8;                                         //Number of columns in our game board.
        public int rows = 8;                                            //Number of rows in our game board.
        public Count wallCount = new Count (5, 9);                      //Lower and upper limit for our random number of walls per level.
        public Count foodCount = new Count (1, 5);                      //Lower and upper limit for our random number of food items per level.
        public GameObject exit;                                         //Prefab to spawn for exit.
        public GameObject[] floorTiles;                                 //Array of floor prefabs.
        public GameObject[] wallTiles;                                  //Array of wall prefabs.
        public GameObject[] foodTiles;                                  //Array of food prefabs.
        public GameObject[] enemyTiles;                                 //Array of enemy prefabs.
        public GameObject[] outerWallTiles;                             //Array of outer tile prefabs.

        private Transform boardHolder;                                  //A variable to store a reference to the transform of our Board object.
        private List <Vector3> gridPositions = new List <Vector3> ();   //A list of possible locations to place tiles.


        //Clears our list gridPositions and prepares it to generate a new board.
        void InitialiseList ()
        {
            //Clear our list gridPositions.
            gridPositions.Clear ();

            //Loop through x axis (columns).
            for(int x = 1; x < columns-1; x++)
            {
                //Within each column, loop through y axis (rows).
                for(int y = 1; y < rows-1; y++)
                {
                    //At each index add a new Vector3 to our list with the x and y coordinates of that position.
                    gridPositions.Add (new Vector3(x, y, 0f));
                }
            }
        }


        //Sets up the outer walls and floor (background) of the game board.
        void BoardSetup ()
        {
            //Instantiate Board and set boardHolder to its transform.
            boardHolder = new GameObject ("Board").transform;

            //Loop along x axis, starting from -1 (to fill corner) with floor or outerwall edge tiles.
            for(int x = -1; x < columns + 1; x++)
            {
                //Loop along y axis, starting from -1 to place floor or outerwall tiles.
                for(int y = -1; y < rows + 1; y++)
                {
                    //Choose a random tile from our array of floor tile prefabs and prepare to instantiate it.
                    GameObject toInstantiate = floorTiles[Random.Range (0,floorTiles.Length)];

                    //Check if we current position is at board edge, if so choose a random outer wall prefab from our array of outer wall tiles.
                    if(x == -1 || x == columns || y == -1 || y == rows)
                        toInstantiate = outerWallTiles [Random.Range (0, outerWallTiles.Length)];

                    //Instantiate the GameObject instance using the prefab chosen for toInstantiate at the Vector3 corresponding to current grid position in loop, cast it to GameObject.
                    GameObject instance =
                        Instantiate (toInstantiate, new Vector3 (x, y, 0f), Quaternion.identity) as GameObject;

                    //Set the parent of our newly instantiated object instance to boardHolder, this is just organizational to avoid cluttering hierarchy.
                    instance.transform.SetParent (boardHolder);
                }
            }
        }


        //RandomPosition returns a random position from our list gridPositions.
        Vector3 RandomPosition ()
        {
            //Declare an integer randomIndex, set it's value to a random number between 0 and the count of items in our List gridPositions.
            int randomIndex = Random.Range (0, gridPositions.Count);

            //Declare a variable of type Vector3 called randomPosition, set it's value to the entry at randomIndex from our List gridPositions.
            Vector3 randomPosition = gridPositions[randomIndex];

            //Remove the entry at randomIndex from the list so that it can't be re-used.
            gridPositions.RemoveAt (randomIndex);

            //Return the randomly selected Vector3 position.
            return randomPosition;
        }


        //LayoutObjectAtRandom accepts an array of game objects to choose from along with a minimum and maximum range for the number of objects to create.
        void LayoutObjectAtRandom (GameObject[] tileArray, int minimum, int maximum)
        {
            //Choose a random number of objects to instantiate within the minimum and maximum limits
            int objectCount = Random.Range (minimum, maximum+1);

            //Instantiate objects until the randomly chosen limit objectCount is reached
            for(int i = 0; i < objectCount; i++)
            {
                //Choose a position for randomPosition by getting a random position from our list of available Vector3s stored in gridPosition
                Vector3 randomPosition = RandomPosition();

                //Choose a random tile from tileArray and assign it to tileChoice
                GameObject tileChoice = tileArray[Random.Range (0, tileArray.Length)];

                //Instantiate tileChoice at the position returned by RandomPosition with no change in rotation
                Instantiate(tileChoice, randomPosition, Quaternion.identity);
            }
        }


        //SetupScene initializes our level and calls the previous functions to lay out the game board
        public void SetupScene (int level)
        {
            //Creates the outer walls and floor.
            BoardSetup ();

            //Reset our list of gridpositions.
            InitialiseList ();

            //Instantiate a random number of wall tiles based on minimum and maximum, at randomized positions.
            LayoutObjectAtRandom (wallTiles, wallCount.minimum, wallCount.maximum);

            //Instantiate a random number of food tiles based on minimum and maximum, at randomized positions.
            LayoutObjectAtRandom (foodTiles, foodCount.minimum, foodCount.maximum);

            //Determine number of enemies based on current level number, based on a logarithmic progression
            int enemyCount = (int)Mathf.Log(level, 2f);

            //Instantiate a random number of enemies based on minimum and maximum, at randomized positions.
            LayoutObjectAtRandom (enemyTiles, enemyCount, enemyCount);

            //Instantiate the exit tile in the upper right hand corner of our game board
            Instantiate (exit, new Vector3 (columns - 1, rows - 1, 0f), Quaternion.identity);
        }
    }

Game Manager:

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class GameManager : MonoBehaviour {

    public BoardManager boardScript;

    private int level = 3;

    void Awake() {
        boardScript = GetComponent<BoardManager>();
        InitGame();
    }

    void InitGame() {
        boardScript.SetupScene(level);
    }

    // Use this for initialization
    void Start () {

    }

    // Update is called once per frame
    void Update () {

    }
}