If you negotiate in GBG (or for daily quests), you need current + previous era. The tech tree requires 2 previous. Other than that, CMap needs a few eras behind but only if you choose to neg rather than attack. Tech tree is the only mandatory to progress... and sometimes daily quests.
You don't need to keep the goods buildings, as long as you've either built up a decent stock (a lot of people use CF for this), or have a supply from trading down (e.g. others in your guild have a lot of those goods and want the ones from your current age).