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Agree. it looks Cheaty and Un-Natural.

But, No, Don't add "Construction Time" on each building 

Because, it cost much for a programmer.

[idea] Just give the Game

{a 1 second "Global Cool Down Time" for all buildings}.

and 2 seconds for "Military" buildings. 

So you can't spamm wall.

E.g. if you build 4 length wall, 

you need to wait for 8 (2x4)  seconds

to build a next Military building.