This looks more like a makeshift Horse pen than a Stable where horses are kept for war.

Also you don't need horses in order to make a stable like building they seem to just add more space for nothing in particular. If one really wanted horses you could add attachment points where the map maker could choose to add horses by adding them in the object editor much like how "MassiveMaster" stable works.

It does really look silly, making the "stables" way bigger giving the illusion that it may house more than 2 ponies would be better.

A few issues to be fixed:

• The texture work at the stone parts is really boring. All faces with same color, same tone, and lacking definition. You need to pick at least 2 or 3 stone textures to use and place them accordingly, so you have rich textures that do value the model.
• That base mesh you've put doesn't look any good in-game. Its texture is too plain and its shape too regular.
  
• The Stand Upgrade First has its selectability kind of broken. You might have to calculate extents, or check collision shapes.

Also, it upgrades with little purpose. The size of the stable remains the same, it only gains a stone fence. You could follow what SNIper of DARKness said regarding its design. (This part is a suggested fix)

Awaiting update.