Ages to our construction went, Dim architecture, hour by hour: And violence, forgot now, lent The present stillness all its power.
Say, is Yoda our speaker now? You might have noticed how many sentences are inverted, with the subject held until after the verb. Don't freak out; this is more than poetic language, for the sake of the rhyme (check out "Form and Meter" for more.) It also adds to a sense of suspension. The moment under the trees is hanging there between afternoon and evening, between one thing and another.
So, it took "ages" for us to be built—that is, to evolve. Still, "construction" also suggests there was human involvement, doesn't it? So does the word "architecture." This isn't just the work of nature, we've crafted ourselves, even if it isn't clear (in fact, it's "dim") exactly what the plan is.
A-ha—we have a colon sighting (okay, chuckle if you must—just get it out of your system), right in the middle of the quatrain.
On the other side is a surprise: violence. It's hard to know what violence the speaker's discussing. There's been no mention of anything violent yet. Maybe he's referring to the kind of original cataclysm or Big Bang that started the whole creation of earth and—waaaay later—humans? Everything is chill now, but that's really thanks to this original violence. It's because of this violence that creation—and our experience of it—even exists (remember the earlier emphasis on just being). If it weren't for the dark, in other words, we wouldn't know about the light.
Finally, we're back to a regular rhyme scheme here, which gives this stanza greater momentum, urging the reader onward.