You can help avoid being raided when you leave a session for the day by spending as much Gold and Eternium as possible. Basically the more resources you have the more attractive your base will be to other players to invade. Even if you can't upgrade units you can still upgrade your mines since they are the only constructions capable of upgrading with Gold. The ideal situation is to leave the game with less than 1,000 Gold and your Eternium in storage.