Ever since the advent of private property, we've had laws to keep uninvited people off that property. Trespassing statutes are some of the oldest, and most vigilantly defended, laws on the books, and they can also have their quirks. Do you have to know you're on someone else's land? Is it trespassing if there are no posted signs? Can you trespass in a store?
And if you've been charged with criminal trespassing, do you need a lawyer to defend the charge?