Its been discussed over various threads. Most illegals come here legally through official ports of entry and overstay their visas. Also, a majority of the land where a wall would be built on is currently private, and often not amenable building due to the topography. Once you get the facts out of the way, you then have to deal with the fact that Trump is doing this for cynical, political reasons and not to protect anyone, so giving in to his demands would be a sort of capitulation to his dog whistle race baiting, which would be terrible policy. In essence, this would be the mother lode of bad ideas.