Haha, I feel silly. I made a defender monk earlier out of the blue, just posted and saw this right under it. I like the idea (maybe because we both thought of it, lol.) My feedback though... Tough Skin is very weak compared to the other bonus action options available to you, imo. You could be taking a dodge action, or a flurry of blows to line up stunning strikes instead compared to d4+Wis THP. For Defender's ki, have the mark last until the end of your next turn to have it line up with Cavalier's Unwavering Mark, not a huge change, just a small consistency thing. I feel Immovable could be a little stronger or maybe broader, 11th is one of the sweet feature levels. Have it extend to more conditions, maybe? Unsure of the exact solution.