acces a field in extended type

I've read this tutorial:
http://www.alsbonsai.com/john/BlitzMax_OOP_Tutorial.pdf
this is one of the examples:

Type TEntity
Field x:Float, y:Float
Method Draw() Abstract
End Type
Type TPlayer Extends TEntity
Field Health:Int
Method Draw()
SetColor 0, 0, 255
DrawOval x, y, 5, 5
End Method
End Type
Type TRobot Extends TEntity
Field Health:Int
Method Draw()
SetColor 255, 0, 0
DrawOval x, y, 5, 5
End Method
End Type
Type TBuilding Extends TEntity
Field Enterable:Int
Method Draw()
SetColor 255, 255, 255
DrawRect x - 5, y - 5, 10, 10
End Method
End Type
Graphics 640, 480
Local EntityList:TList = New TList
Local obj:TPlayer = New TPlayer
obj.x = 5
obj.y = 7
EntityList.AddLast(obj)
Local obj2:TBuilding = New TBuilding
obj2.x = 15
obj2.y = 3
EntityList.AddLast(obj2)
'Draw all entities
Local ent:TEntity
For ent = EachIn EntityList
ent.Draw()
Next
Flip
WaitKey

But I don't understand how I can access the field "enterable" in the buildingtype, while going through the list.
A bit of help please?