SharePoint Layout Page for Only Administrator Accessible

SharePoint Layout (Application) pages are accessible by any user who has a least possible access/permissions for any of the sites in the server/farm.

A layout page access can be restricted to only Site Administrator by overriding the RequireSiteAdministrator property of LayoutsPageBase class.

The below code snippet shows sample implementation.

using Microsoft.SharePoint.WebControls;
public class MyLayoutPage : LayoutsPageBase
{
  //It mmakes sure only site administrators can access this page
  protected override bool RequireSiteAdministrator
  {
    get
    {
      return true;
    }
  }
  protected void Page_Load(object sender, EventArgs e)
  {
    //Code goes here...
  }
}

It is just one of the members, look at the post here to know about many other members/methods of LayoutsPageBase class.