I suggest that you show an attorney exactly what you intend to install and what the attorney recommends to reduce your risk of liability. It appears CA has departed somewhat from the old common law rules (still used in my state and others) regarding trespassers. A homeowner does not owe a trespasser the same duty of care that he/she would owe to others that are on the property with the owner's consent, but there is still a duty to either make safe or to warn others of known dangers on the property. Exactly what the minimum required is varies depending on the situation, like how apparent the danger is to others even without a warning or notice of some kind, how the person enters (or tries to enter) the property, etc. A no trepassing sign may be helpful in dealing with trespasser, but a statement that you are "not responsible for injuries" may not be enough to put a person on notice of any significant known hazards. You might need something more specific like "warning, dangerous spikes on top of the fence" That's something you want to ask the attorney about. You also need to consider what risk of harm your spikes might pose to others, like children for example. The exact design of the fence, how high the fence is, and what, if anything, you might have on your property that would motivate children to try to get over the fence are all factors to consider.
You'd also want to discuss it with your homeowners insurance carrier (hopefully you're not in an insurance desert where there is no insurance to get, which is becoming a more prevalent problem). Even if what you do would likely help you avoid liability, you want to be sure that your insurance will pay should you get a judgment against you. Having adequate warning, etc., helps to reduce your liability, but the trespasser may sue you anyway and litigation can be expensive even if you prevail. If you are covered for it, however, the insurer will provide the legal representation for you at its expense.
