What is Cao Cao's plan of watching fire from across the bank?

At the end of the Eastern Han Dynasty, Cao Cao and Ma Chao fought at Tongguan. When the battle began, Cao Cao relied on his strong troops and horses and took the lead, hoping to defeat Ma Chao in one fell swoop. Unexpectedly, Ma Chao was extremely brave, and the Xiliang soldiers he led were all brave and good at fighting, and they rushed over with Ma Chao. In the Battle of Tongguan, Cao Cao's army suffered heavy losses and was severely weakened. Cao Cao had to pack up the remaining troops, cross the Wei River, and reach the north bank. Ma Chao's officers and soldiers took advantage of the victory to pursue and stationed themselves on the Wei River, cutting off Cao Cao's transportation and making it difficult for the officers and soldiers to set up camp for a while. Cao Cao knew that if he did not set up camp quickly, Ma Chao might launch an attack at any time, and the army without a camp would be vulnerable. However, there is no ready-made city near the north bank for camping. For this reason, Cao Cao was extremely anxious. At this time, one of Cao Cao's staff suggested to him: "Prime Minister, we can use the soil from the Wei River to build an earth city." Cao Cao thought it was reasonable, so he mobilized 30,000 troops and asked them to carry earth to build the city. However, the soil in the Weihe River is full of sand. As the saying goes, a tower can only be built with sand but only three feet. The sand and soil of the Weihe River are so loose that no wall can be built. In addition, Ma Chao's soldiers and horses often came to harass him, which made Cao Cao worried day and night. It was early winter, and for several days it was covered with dark clouds, and sometimes it rained. All this tells people that severe cold is coming. On this day, Cao Cao was worried, and suddenly he heard a hermit asking for an audience, and said to Cao Cao: "The Prime Minister has been building a city and setting up camp, why is he so slow to do so?" Cao Cao sighed and said: "This is all sandy soil, and earthen cities are built as time goes by. How can we build it?" The hermit smiled and said, "The Prime Minister has always been a master of military affairs. Don't you know that the weather has changed? It's cloudy these days. The north wind is blowing tonight. The weather will be extremely cold, and if you build a city with Weihe mud, it will surely freeze as you build it." Cao Cao suddenly frowned and immediately ordered his soldiers to make all preparations. That night, as expected, the north wind was strong and the temperature dropped sharply. Cao Cao was overjoyed and braved the severe cold to command all the soldiers to carry earth and build the city. He piled the earth, watered it, and froze it layer by layer. By the time dawn came, the earth city had been built. Ma Chao's spies saw Cao Ying's Tucheng standing and immediately went to report to Ma Chao.

Ma Chao went to inspect it in person and was surprised. He thought, how could Cao Jun transform an earthen city overnight? Could it be that gods were helping?