The most important factor when building a base is to make sure it has been designed in a way that it can be easily defended. Place barricades where they will slow the zombies down and have towers behind those barricades but in front of the three buildings you must defend. Make your first priority leveling up your headquarters in order to unlock more buildings and then level those up whenever possible. Always repair any damaged buildings on your base so you have as many things as possible in the way of the advancing zombies. Make sure you work on removing the zombie nodes whenever you have the chance as doing so gives you a huge bonus. Defending your base is a mission that you can attempt every 4 hours and it is something that you should always consider doing because you can earn easy resources from it. The mission itself is not that difficult as the three buildings you must defend have plenty of hit points.