The best way to protect a Christmas tree from a cat is to make sure it's securely anchored to the ground or the wall, so it doesn't tip over. You can also use a tree skirt to cover the base of the tree, which will help deter the cat from climbing it. You should also avoid using breakable or fragile ornaments, and instead, opt for sturdy, shatter-proof ornaments that won't break if knocked down. Finally, you can use a spray bottle with water or citrus oil to spray the cat if it gets too close to the tree.