Resources are the most important

You get plenty of speed-ups, so it's better not to spend diamonds on them. It's actually better to spend diamonds on resources. Maybe it was good timing with the event, but I reached base level 21 on day 7 (though I started about 2 hours before the day changed, so it was actually around day 6). It's a common castle-building game, but resources are by far the most lacking thing.