Note to Rage Quitters: It's not GG once the other team gets Giants.
In fact, if you go back to your citadel and defend it from said giants, you will level EXTREMELY quickly. On more than one occasion, my team has been able to push back from the brink of death and completely overhwlem the opposing team since their giants let us level way past them.
This leads directly into another important idea: Don't buy giants until you are ABSOLUTELY certain that you will be able to destroy the opposing team's citadel in one swift push. If you buy giants and you aren't able to destroy the citadel swiftly, the opposing team will likely outlevel you by a massive degreee and crush you.