I wrote "marco miliario" as sugested in the translation and the answer is just 'marco'
Is "marco" only used in the literal sense, ie in orienteering? Or could it also mean a milestone in history, in a project and so on?
Yes, it can be used in many senses, as the ones you mentioned =)
is the order of words formal or informal? I said " E um quito importante marco" . Is this wrong?
I feel like "meta" should be accepted, depending on the context. Many times in English when we say milestone, we mean it interchangeably with goal