Gaining access to upper Minas Tirith
Impossible? Read on!
So, first, you must create a hero with the power to transport units to the best tier possible. Once you do, start a battle on Minas Tirith in which you control the White City. Level up your hero to his best transport, and select a builder; using this method, you can transport a builder to the third tier of the White City, which is normally inaccesible. Then, you can build a structure that builds siege units up there.
You will discover that from that high up, your siege weapons can fire an awe-inspiring distance, and obliterate enemy emplacements within a good radius around Minas Tirith; this forces your foe to stay as far back as possible. It's also pretty fun!