Hence, there needs to be a method which can simplify the task of destroying even the strongest defense line. At the same time, not comprimising the viability of light defensive build where players choose to forgo the early game and prepare themselves for the mid/late game.
That's resolved by designing a rock-paper-scissors system to the game. The difference between successful RTS games and failed ones are usually whether the developers understand basic war theory, such as the war machine triangle (Defense > Attack > Production > Defense). Not to mention the psychology behind successful warfare (e.g. making the opponent weaker by means of deception and the fear of uncertainty).
But don't get too self-assured either. The reason why a person fails to beat a turtler could just be because he's bad at it, too. I.e. unrelated to the game design. In Starcraft 2, for instance, there's practically zero difference in viability between defensive, offensive and production strategies. They all cancel eachother out, not to mention the fact that the inferiority of a tactic can be eliminated by exploiting not only an opponent's weaknesses, but his strengths as well).
Be sure to confirm whether your issues with the game is a common trend or if it's just you.