Here's my tactic: If the map is XL, I gather resources and build up very quickly. When strong enemies arrive, it's best to meet them in the castle, using the towers for cover, which allows you to win with minimal losses. It's easier that way. Then, when there are fewer strong enemies left, it's better to quickly buy out the remaining troops and capture enemy castles before they have a chance to build up a large army. If the map is S or M, it's better to gather resources and defeat monsters for experience, then stay in the castle and repel attacks until a large army is accumulated. And when it's accumulated, you should also try to quickly capture enemy cities. I started with this tactic.