you need to be at least level 20. To gain diamonds you can spend real money but i suggest doing quest and hall of hero battles to gain them they both quit easy to do
at level 25 you can buy a fortification workshop for 28k gold, this way cost coins
go to shop
go to base , doing it this was will cost you diamonds
i suggest using watch towers they cost 5 diamond each
i also can confirm you have to click on the one's that have a shield on the bottom right of the picture
watch tower 5 diamonds
guard post 15 diamonds
archer tower 30 diamonds
citadel 20 diamonds
fort 70 diamonds
shouldn't be able to down vote without giving a solid reason why