routes.MapPageRoute("mypage", "mypage/{mypageName}", "~/mypage.aspx");
This will work only for URLs in this format:
www.example.com/mypage/changingparthere
If you want to make it
www.example.com/changablemypage
Set it to:
routes.MapPageRoute("mypage", "{mypageName}", "~/mypage.aspx");
But as you can see, it will catch literally everything. So make sure it is the last routing on Global.asax.