1N21 climbs from the Shelton Trail
(small city/county trail network ) and the water tank at the end of Van Leuven to a wide doubletrack designated as City Creek Road (1N09). The surface is generally hardpack and features almost nothing technical. The 2.0 miles or so at the north end is quite scenic and nicely shaded. At its peak, the trail tops out at ~3,200ft elevation, then descends to about 2,600ft as you approach 1N09. The finale is a small creek crossing littered with shotgun shells.
At 1N09, you can turn left and head to CA330 for a paved descent back into Highland but it's not really worth it as the scenery isn't so interesting. Also, note that CA330 has no shoulder and a guardrail that crowds the edge of the road thus it isn't too safe for cyclists. I recommend turning around and descending this trail.
Ultimately, this is a low traffic trail with an occasional hiker near the bottom. I've marked this as intermediate due to short steep sections and occasional narrow bits with near-vertical dropoffs but otherwise easy to ride.