Siege Master Achievement
Destroyed the Keep 100 times in Multiplayer Battles
This might actually be one of the easier achievements to earn early on in the game. For this achievement, you need to focus on taking down the keep which will automatically give you a win and some crowns. Units like the trebuchet tend to demolish the keep fairly quickly. The best strategy that I can recommend for something like this is to take out defensive towers and send in things like the trebuchet to take down the keep from a distance. Should come fairly easily if there aren't any traps.