So, at the moment I'm reading Caesar's Gallic War as good-night reading (wherefore my Greek professor told me I was mad), and somehow it came to me last night: imperfect praeteritum (I don't know the proper English term, I'm afraid, but the -ba- forms) is often described as an action in the past not done (hence im-perfect).
This definition seems to me an alteration of actual usage, as the only strictly speaking imperfective use of the tense would be the relatively rare praeteritum de conatu, which I don't recall ever having met in a text; the most common is the iterative use.
When used iteratively, my gut feeling tells me the action is looked upon as ongoing in the past (Caesar in Gallia proelium gerebat annos VIII, for example) but not still going on at the time of speaking.
At the same time, the historical perfect is aoristic, again my feeling, as in Caesar pontem fecit trans Rhenum - the construction of said bridge took 10 days (bell. gall. IV, 17), but looked upon in the perfect tense/aspect, the important thing is that the bridge was built and done, so that the legions could cross the river and fight the barbars.