I was defeated by a player that had less power, how is that possible?

Please note that the overall power of your Castle does not determine the strength of your Troops. Taking into account the myriad of ways it is possible for Troops to be strengthened, it is possible to conquer a much larger opponent or in turn be conquered by a much smaller opponent depending on how they have spent their time researching upgrades for their Troops. Additionally, Troop types are leveraged against each other, if your Castle for example is defended by a large number of the same type of Troop, it is possible that your walls will be overrun if a weaker Lord attacks you with leveraged Troops.