how to create counter using asp.net?

You can use timer controls of ASP.net Ajax Extension and on the Tick event of timer you can update your counter. But since it's a server side control so I don't suggest it to use in this case as it will reduce the response time of page.
Instead I suggest use the javascript or JQuery to count and update the timer and when timer comes to zero then raise a server side event to stop the game.

Below is the script:

  var count = 5;
  $(document).ready(function () {
  var timerVal = $('#lblMessage').text();
  if (timerVal != 'Counter finished') {
  $('#counter').text(count);
  window.setTimeout(timer, 1000);
  }
  });

  function timer() {
  $('#counter').text(count);
  if (count == 0) {
  __doPostBack('LinkButton1', '')
  }
  else {
  count--;
  window.setTimeout(timer, 1000);
  }
  }

And HTML


  LinkButton
 

Hope this will help you.