First you click on the Arrow in the bottom right hand corner to Edit  ypur Castle 

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Then go to click on the  protected Lands in the top right  of the screen 

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That will show all your  stored items 

It's  mostly used to store seasonal items 

There was a glitch at one time  that some exploited where  players  stacked  buildings on top of each other but I think it's been corrected.  

Thanx Wesley, I couldn't get a building stored then tried a castle improvemnt and it worked.