How to properly create a 2 dimensional array of an object. [C#]

No errors in mono develop, but the console gives this for line 34:

error CS0178: Invalid rank specifier: expected ,' or ]'

I'm trying to create a List that holds 2 dimensional arrays of my placeholder objects. The object class just holds information in variables.

Here is my object class:

 using UnityEngine;
 using System.Collections;
 
 public class placeholder{
 
     public static bool exists = false;
     public static bool initialized = false;
     public static int priority = 0;
 }

Here is my entire script I'm working on; just in case. I suspect only line 34 for will be relevant however.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class floorbuilderv2 : MonoBehaviour {
 
     //room related
     public GameObject[] fourrooms = new GameObject[3];
     public GameObject[] threerooms = new GameObject[3];
     public GameObject[] tworooms = new GameObject[3];
     public GameObject[] onerooms = new GameObject[3];
     public GameObject[] startrooms = new GameObject[1];
 
     //flooor related
     int currentfloor = 0;
     public int Size_Increase_Factor = 5;
     List<placeholder[][]> existanceplaceholders = new List<placeholder[][]>(); 
  
     //placeholder[][] existance = new placeholder[10][10]; 
 
     //misc
     void Start () {
         transform.position = new Vector3 (0, 0, 0);
         buildnextfloor ();
     }
     
     void buildnextfloor(){
         int gridsize = currentfloor * 10;
         int starttile = Mathf.FloorToInt (gridsize/2);
         int roomquantity = (currentfloor + currentfloor) * Size_Increase_Factor;
         int roomsinitialized = 0;
         int priorityfactor = 1;
 
         placeholder[][] newplaceholder = new placeholder[gridsize][gridsize] ;
 
         existanceplaceholders.Add (newplaceholder); 
 
         creattile (starttile, starttile, 4, priorityfactor);
         priorityfactor ++;
 
         while(roomsinitialized < roomquantity){
         
             //int[] xcue = new int[gridsize];
             //int[] ycue = new int[gridsize];
 
             for(int i = 0; i < gridsize; i++){
                 for(int ii = 0; ii < gridsize; i++){
                     if(existanceplaceholders[currentfloor][i][ii].exists == true && existanceplaceholders[currentfloor][i][ii].initialized == false && roomsinitialized < roomquantity){
                         creattile(i,ii,0,priorityfactor);
                     }
                 }
             }
         }    
         Debug.Log ("test");
         Debug.Log ("test");
     }
     
     void creattile(int x, int y, int staticdoorcount, int priority){
         existanceplaceholders[currentfloor][x][y].exists = true; 
         existanceplaceholders[currentfloor][x][y].initialized = true;
         existanceplaceholders [currentfloor] [x] [y].priority = priority;
         int adjacents = 0;
 
         roomsinitialized ++;
 
         if (staticdoorcount > 0){
             if(staticdoorcount = 4){
                 existanceplaceholders[currentfloor][x+1][y].exists = true;
                 existanceplaceholders[currentfloor][x][y+1].exists = true;
                 existanceplaceholders[currentfloor][x-1][y].exists = true;
                 existanceplaceholders[currentfloor][x][y-1].exists = true;
             }
         }
 
         if ((int)Random.Range(1, 3) == 1){
             adjacents = 1;
         }
         else if((int)Random.Range(1, 3) != 1){
             adjacents = 2;
         }
         else{
             adjacents = 3;
         }
 
         int offsetx = 0;
         int offsety = 0;
 
         if(adjacents == 1){
             int attempts = 0;
 
             offsetx = setoffsetx();
             offsety = setoffsety(offsetx);
 
             while(existanceplaceholders[currentfloor][x +offsetx][y+offsety].exists == true){
                 offsetx = 0;
                 offsety = 0;
 
                 offsetx = setoffsetx();
                 offsety = setoffsety(offsetx);
 
                 attempts ++;
 
                 if(attempts > 3){
                     break;
                 }
             }
 
             existanceplaceholders[currentfloor][x +offsetx][y + offsety].exists = true;
         }
         else if(adjacents == 2){
             for(int i=0; i<2; i++){
                 int attempts = 0;
                 
                 offsetx = setoffsetx();
                 offsety = setoffsety(offsetx);
                 
                 while(existanceplaceholders[currentfloor][x +offsetx][y+offsety].exists == true){
                     offsetx = 0;
                     offsety = 0;
                     
                     offsetx = setoffsetx();
                     offsety = setoffsety(offsetx);
                     
                     attempts ++;
                     
                     if(attempts > 3){
                         break;
                     }
                 }
                 
                 existanceplaceholders[currentfloor][x +offsetx][y + offsety].exists = true;
             }
         }
         else if(adjacents == 3){
             for(int i=0; i<3; i++){
                 int attempts = 0;
                 
                 offsetx = setoffsetx();
                 offsety = setoffsety(offsetx);
                 
                 while(existanceplaceholders[currentfloor][x +offsetx][y+offsety].exists == true){
                     offsetx = 0;
                     offsety = 0;
                     
                     offsetx = setoffsetx();
                     offsety = setoffsety(offsetx);
                     
                     attempts ++;
                     
                     if(attempts > 3){
                         break;
                     }
                 }
                 
                 existanceplaceholders[currentfloor][x +offsetx][y + offsety].exists = true;
             }
         }
     }
 
 
 
     int setoffsetx(){
         if(Random.Range(1,2) == 1){
             int rnd = Random.Range (1, 2);
                 
             if(rnd == 1){
                 return -1;
             }
             if (rnd == 2) {
                 return 1;
             }
         }
         else{
             return 0;
         }
     }
     int setoffsety(int offsetx){
         if(offsetx == 0){
             int rnd = Random.Range (1, 2);
             
             if(rnd == 1){
                 return -1;
             }
             if (rnd == 2) {
                 return 1;
             }
         }
         else{
             return 0;
         }
     }
 
 
     void Update () {
     
     }
 }