Occasionally closed in muddy/wet conditions. Popular with hikers, and Jefferson County rangers have been known to ticket riders who exceed 15 MPH or ride aggressively.
From the Mt. Falcon parking lot, you'll get a minimal warmup before the long grind on the Castle Rock trail begins. The grade isn't too steep but there are enough water bars, tight switchbacks and rock fields to make this a demanding climb.
About halfway up, a hard lefthand turn (with just a touch of tree shade) offers a brief respite before the climbing kicks in again. Then it's more grinding in small gears to the top. It's challenging to stay on the bike the entire time not because the technical challenge is super high but simply because of the relentless climbing and moderate obstacles that just keep coming.
A fit rider keeping the hammer down will need about 30 minutes (give or take) to reach the stone shelter. At a conversational pace, or with some breaks, the ascent could take an hour or more.
This trail continues all the way down to the other parking lot on the west side of the system at Mt. Falcon road, but most use Castle Trail to connect to the other great singletrack in the area.
From the stone shelter, continue on the Meadow trail to reach Parmalee and other Mt. Falcon singletrack. Or turn around at the shelter for the downhill leg just keep an eye peeled for uphill traffic and give yourself enough time to brake/yield on the sometimes loose surface.