About.com has good descriptions about the differences for temporal prepositions. Check it out if you're trying hard to grasp the minute differences like I have been.
Although "des siécles des siécles" (or something very similar) is used in prayers and the like to mean "forever and ever" or "for ages and ages". That is the usage I am most familiar with, but "ages" was not accepted. Duolingo provides "age" as one of the possible translations for siécle, so it should be accepted, I think.
Well "In" and "Within" do have differing connotations. The contexts, though, are far too diverse for there to be any real semblance here in my mind. I may be wrong, I often am, but I think that in this task "In" and "Within" should suffice. Your solution should be accepted. There is no context. For me, One thing may have happened "In the century". OK? But many things may have happened at different times "Within the century". So, with such a lack of context your solution should have been accepted. Duo can not Have its cake and Eat it Jh4. Have lingots. JJ.
"En un siècle" means that it takes a century, as in "Je pourrais le construire...en un siècle" ("I could build it...in a century", i.e. it would take me a century"). "Dans un siècle" means that it will happen after a century has passed, as in "Le réchauffement de la terre va nous tuer dans un siècle" ("Global warming will kill us in a century" or "a century from now").