Once you have an object setup (size, name, color, skinned with an image, etc), you can easily duplicate it by holding down the "Ctrl" key (the control-key) and using the mouse - drag off new copies from the original. Then, if you need to, rename the copies.

Manually - 

        After you have your object (button, card, etc) setup, you can clone it in the message box:

                              

Alternatively, you can copy images by holding down the "Ctrl" or "Cmd" key and drag off a new copy. Then rename the copy to a new name of it's own

You can create objects inside the game. As the game goes on, you can add more and more enemies to make it harder. Have them multiply and make the game get harder as it goes along.
Suppose, you decide to generate enemies at random. It would make sense to write a handler to do it, and call that handler at random times.

The following is a sample of code that you might use:

    global y

    on makeEnemy

        clone button "EnemyX"

        set the name of the last button to ("enemy" & y)

        add 1 to y

        set the loc of last button to  random(400),random(400)

        set the visible of the last button to true

    end makeEnemy

 where:

        We assume that you have a sample enemy button called "EnemyX"  off the screen that you will use to make copies from

        We use the variable "y" as a counter to keep track of the number of enemies we have created

        We have it appear at a random location (assuming our page is 400 pixels wide and 400 pixels high)

    

You can create new ones by dragging them off the original

global myLoc
on mouseDown
   grab me 
   put the loc of me into myLoc
   clone me
   set the name of me to ("b" & y)
   set the loc of image "Tower" to myLoc
end mouseDown

or

global y,x
on mouseDown
   grab me
   clone me
   //clone btn "box"
   set the name of me to ("b" & y)"
   //set the loc of btn "box" to the loc of btn ("box" & y) 
   add 1 to y
    //grab me
end mouseDown