As Samuel returned to Zarahemla, the crowds apparently recognized him and would not allow the Lamanite to enter the city. Samuel's only alternative was to somehow climb to the top of the city wall. This wall was made by Captain Moroni for the specific purpose of being difficult to climb but somehow Samuel got up to the top and began to preach.
Of course he would want to be heard by as many people as possible, so it is likely that he deliberately mounted the wall near the city gate where the major market was usually located.
Shouting at the top of his voice, Samuel proclaimed his message.