Guard is a synonym of Fence. Fence is a type of Wall. Wall is a type of Difficulty. 
you are likely to find Lobby in/at/on Opera. you are likely to find Electricity in/at/on Opera. Electricity can be Powerful. 
Guard <--Synonym--> Fence --IsA--> Wall --IsA--> Difficulty 
Lobby --AtLocation--> Opera <--AtLocation-- Electricity --HasProperty--> Powerful 