Problem drawing 2D on 3D

using System;
using System.Drawing;
 
using OpenTK;
using OpenTK.Input;
using OpenTK.Graphics;
using OpenTK.Graphics.OpenGL;
 
namespace Julia3D {
    class JuliaWindow : GameWindow {
 
        public Camera camera;
 
        private float x = 0.0f, z = 0.0f;
 
        public JuliaWindow ()
            : base (800, 600, GraphicsMode.Default) {
 
            Title = "Julia Window Test - " + GL.GetString (StringName.Version);
            VSync = VSyncMode.On;
 
            Keyboard.KeyRepeat = true;
 
            camera = new Camera ();
 
        }
 
        protected override void OnLoad (EventArgs e) {
            base.OnLoad (e);
 
            GL.ClearColor (Color.Black);
            GL.Enable (EnableCap.DepthTest);
 
        }
 
        protected override void OnResize (EventArgs e) {
            base.OnResize (e);
 
            GL.Viewport (ClientRectangle.X, ClientRectangle.Y, 
                         ClientRectangle.Width, ClientRectangle.Height);
 
            Matrix4 projection = Matrix4.CreatePerspectiveFieldOfView (MathHelper.PiOver4, Width / (float) Height, 1.0f, 64.0f);
            GL.MatrixMode (MatrixMode.Projection);
            GL.LoadMatrix (ref projection);
        }
 
        protected override void OnRenderFrame (FrameEventArgs e) {
            base.OnRenderFrame (e);
 
            GL.Clear (ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
            Matrix4 projection = Matrix4.CreatePerspectiveFieldOfView (MathHelper.PiOver4, Width / (float) Height, 1.0f, 64.0f);
            GL.MatrixMode (MatrixMode.Projection);
            GL.LoadMatrix (ref projection);
            camera.Render ();
 
            GL.Translate (0.0f, -0.5f, -6.0f);
            GL.Scale (3.0f, 1.0f, 3.0f);
 
            float size = 5.0f;
            int linesX = 30;
            int linesZ = 30;
 
            float halfsize = size / 2.0f;
            GL.Color3 (Color.White);
            GL.PushMatrix ();
            {
                GL.Translate (0.0f, -halfsize, 0.0f);
                Utils.DrawNet (size, linesX, linesZ);
                GL.Translate (0.0f, size, 0.0f);
                Utils.DrawNet (size, linesX, linesZ);
            }
            GL.PopMatrix ();
            GL.Color3 (Color.Blue);
            GL.PushMatrix ();
            {
                GL.Translate (-halfsize, 0.0f, 0.0f);
                GL.Rotate (90.0f, 0.0f, 0.0f, halfsize);
                Utils.DrawNet (size, linesX, linesZ);
                GL.Translate (0.0f, -size, 0.0f);
                Utils.DrawNet (size, linesX, linesZ);
            }
            GL.PopMatrix ();
            GL.Color3 (Color.Red);
            GL.PushMatrix ();
            {
                GL.Translate (0.0f, 0.0f, -halfsize);
                GL.Rotate (90.0f, halfsize, 0.0f, 0.0f);
                Utils.DrawNet (size, linesX, linesZ);
                GL.Translate (0.0f, size, 0.0f);
                Utils.DrawNet (size, linesX, linesZ);
            }
            GL.PopMatrix ();
 
            GL.Scale (1.0f / 3.0f, 1.0f, 1.0f / 3.0f);
 
            GL.Translate (x, 0.0f, z);
            GL.Color3 (Color.Navy);
            Utils.DrawSphere (1.0, 100, 100);
            Utils.DrawCube (1.0f);
            // Finish rendering
            GL.Flush ();
 
            SetupOrtho ();
 
            SwapBuffers ();
 
        }
 
        private void SetupOrtho () {
 
            GL.Disable (EnableCap.DepthTest);
 
            GL.MatrixMode (MatrixMode.Projection);
            GL.PushMatrix ();
            GL.LoadIdentity ();
            GL.Ortho (0, Width, Height, 0, -1, 1);
            GL.MatrixMode (MatrixMode.Modelview);
            GL.PushMatrix ();
            GL.LoadIdentity ();
 
            Draw2D ();
 
            GL.MatrixMode (MatrixMode.Projection);
            GL.PopMatrix ();
            GL.MatrixMode (MatrixMode.Modelview);
            GL.PopMatrix ();
            GL.LoadIdentity ();
 
            GL.Enable (EnableCap.DepthTest);
        }
 
        private void Draw2D () {
 
            GL.Translate (10, 10, 0);
            GL.Color3 (Color.Olive);
            GL.Begin (BeginMode.Quads);
            {
                GL.Vertex3 (0, 0, 0);
                GL.Vertex3 (0, 100, 0);
                GL.Vertex3 (100, 100, 0);
                GL.Vertex3 (100, 0, 0);
            }
            GL.End ();
        }
 
        [STAThread]
        static void Main (string[] args) {
 
            using (JuliaWindow game = new JuliaWindow ()) {
                game.Run (30.0);
            }
 
        }
    }
}