Leaf City is located at the southernmost tip of the Northern Territory, where the three domains converge. It borders the Eastern and Southern domains, is close to Spring Country, and is also one of Summer Country's more developed trade cities.
The key point is that the city spans a vast area and has a large population. According to Coarmy's information, there are at least several million people living there, so attempting to rescue them is no small feat.
The only silver lining is that the military strongholds of the Northern Territory group are located around Leaf City. Four divisions of troops, approximately 50,000 soldiers, are stationed there.
When the game apocalypse first began, soldiers from the military strongholds were dispatched to Leaf City to form a defensive line, which barely managed to slow down the spread of the crisis.
If they hadn't arrived in time, Leaf City would likely have fallen already.