You can increase your Prestige by upgrading buildings, training army and winning battle. Once you lose city or army get defeated, your Prestige will decline. To some extent, a lord's Prestige can decide his weight in this world. If you possess high Prestige, gangsters are daunted with your name and dare not to lay eyes on your cities. The most important is that, with the aggregations of your Prestige, you're more likely to be granted with higher ranking, which enables you to rule more cities.