```lisp
(asdf:load-system :cl-cffi-gtk)

(defpackage :gtk-tutorial
  (:use :gtk :gdk :gdk-pixbuf :gobject
   :glib :gio :pango :cairo :common-lisp))

(in-package :gtk-tutorial)

(defun example-simple-window ()
  (within-main-loop
    (let (;; Create a toplevel window.
          (window (gtk-window-new :toplevel)))
      ;; Signal handler for the window to handle the signal "destroy".
      (g-signal-connect window "destroy"
                        (lambda (widget)
                          (declare (ignore widget))
                          (leave-gtk-main)))
      ;; Show the window.
      (gtk-widget-show-all window))))
```