From the Mt. Falcon parking lot, you'll get a minimal warm up before the long grind on the Castle Rock Trail. There is no protection from the sun on most of this trail, so avoid hot days. This trail can be very crowded with runners, hikers, mountain bikers, and even horses.
About halfway up, a hard left turn (with just a touch of tree shade) offers a brief respite before the climbing kicks in again. Then it's more grinding to the top.
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 trails in the area. The views along the top of this trail are quite good, extending from Kansas to the continental divide!
If it's crowded or you want to avoid bikes, consider using Turkey Trot
instead of this trail - they reconnect two-thirds of the way up the hill.