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Well, it looks like you need this sign:

https://a.d-cd.net/10c55e8s-960.jpg

ТЕРРИТОРИЯ МИНИСТЕРСТВА ОБОРОНЫ
ПРОХОДА НЕТ!

"Territory of Defence Ministry"

"installation" or "object" will...

And again you should provide context: where it is written.
Literally: "ВОЕННОЕ СООРУЖЕНИЕ", but it sounds strange in most situations. I think "ВОЕННЫЙ ОБЪЕКТ" will be more suitable in much more...