Premises Liability (to a Trespasser)
Premises liability is a common-law tort, but in most states (including Colorado) it has been replaced by statute. The Colorado Premises Liability Statute is C.R.S. § 13-21-115, and is the exclusive means for relief against the landowner (meaning that a parallel negligence cause of action normally will not lie, though other claims outside the Premises Liability Statute may be available against other defendants on other theories).
What duty do you owe a trespasser? In Colorado, the elements of liability under the Premises Liability Statute for a trespasser under 13-21-115(3)(a) are:
(1) The plaintiff had damages;
(2) The defendant acted willfully, or deliberately; and
(3) The defendant’s willful or deliberate conduct was a cause of the plaintiff’s damages.
Bottom line, it's probably very unwise (for a number of reasons) to set up a Punji-pit in your front lawn to impale someone who may walk across it at night. But if an adult sneaks into your swimming pool and drowns, you are likely safe.
In stark contrast, where the person injured was a licensee, invitee, or child onto the land, something approaching a negligence standard (i.e. failure to use reasonable care by the landowner) will apply.