What is the best strategy for defense. I'm at Level 11. What are the best troops to defend the base...

Try joining an alliance and increasing your troop power. I stayed as a farmer until level 16 to help with resource protection. Also, use vehicles for added speed and payload improvement. There is no real good way to defend against higher level players, but the less they can plunder means the more you can level up; hence staying a farmer until level 16. Focus on vehicles and upgrading your class apc for longer term success.