Creating linked duplicates?

I have an object that I will need to create many copies of, (They are buttresses for my castle if your interested), but I don’t want to just ctrl+d them, cause I don’t want to have to hand change any changes I need to make. So I was wondering if there was any way to link all the duplicates to one model, so that when I change that one they all change?

For some reason I have a vague recolation of a function like this but I can’t remember enough to actually find any info on it. Thanks.

Alt+D is the answer