You brought up a big hole in the timeline that I've wondered about sometimes, and that is, how did the Forerunners have time to build the rings if the Flood was nagging them. I've tended to assume that the time between first finding the flood nd building the rings wasn't that long. They were really advanced and could probably do such a feat in a small amount of time. And they discovered the spores when they were mining in gas giant, right? So they must have been able to contain them somehow without the Halos. Colossus' gas canisters...

Bad word usage by me, by installation I basically meant "facility," I wasn't speaking of a Halo ring

And Ibeechu, I think that there were a few centuries between initial contact and activation of the rings, and my guess is that the were built 250-ish years into the fight

-Elpolloguapo

This may be obvious (and therefore unmentioned thus far), but there are numerous previous Halo maps referenced in this WWU segment. Containment from Halo 2, was a snowy level (plus "lowering ambient temperatures", etc.), Derelict from Halo:CE, remade in Halo 2 as Desolation. We also saw 2401 PT floating around and humming in Halo 2's Backwash, which was a Flood-themed level. Could it be a big ol' pack of remade maps?

300 years passed from first contact with the Flood to the creation and deployment of Mendicant Bias. From there, it was 43 years until Mendicant made his decision to turn.

350 years is plenty of time to come up with the idea and construct the installations.

This topic is unfortunately going very far away from the Terminals and even this most recent message into Forerunner history and such. While that's fine, I think people are forgetting the novels (specifically Ghosts of Onyx) and how truly advanced the Forerunner are. Considering what Onyx is made of and the fun little thing that our heroes end up going to at the end, it's beyond a shadow of a doubt that the Forerunner are supreme and awesome (except when it comes to fighting off evil galaxy sized plagues).