Players climb onto and up it with 'climb climbable', and descend with 'descend climbable'.
@height climbable is  
  Used by the climb verb to detect when you can't go any further. 
@describe climbable at level as string  : Defines a single line description 
  to be printed to the player when e climbs or descends to that level, or 
  to be appended to the output of the containing room's :look_self when  
  executed by the player.  (This is an awful kluge.  Don't read the code, OK?) 
  No editing is available; you may only replace strings with @describe again. 
@capacity climbable is  
  can be climbing on the climbable at once. 
 
@messages available to set: 
  @next-step climbable is "You climb up." 
  @onext-step climbable is "climbs up the %d." 
Printed to player and room (respectively) when someone climbs from one level to the next. 
  @start-climbing climbable is "You climb onto the %d." 
  @ostart-climbing climbable is "climbs onto the %d." 
Printed to player and room (respectively) when someone climbs onto the climbable for the first time. 
  @step-down climbable is "You descend the %d." 
  @ostep-down climbable is "descends the %d." 
Printed to player and room (respectively) when someone descends from one level to the previous. 
  @stop-climbing climbable is "You get off the %d." 
  @ostop-climbing climbable is "gets off the %d." 
Printed to player and room (respectively) when someone descends from the bottom level to the floor 
 
Programmatic Customization: 
The verb :look_height is called in all places where the climbable wants to print out one of the height descriptions entered with @describe above.  This verb does player:tell; it doesn't return its string or anything.  You can override it, or add stuff to it. 
 
The verb :set_description(level,description) is used by the @describe verb, and can be used to set multiline descriptions by making the second argument a list. 
  The Generic Climbable Object is invitingly empty.