As IMP at this level, use 6 mortars and 2rockets. The mortars will kill rebel infantry load (common at this level) and the rockets are to protect against hovers. (jugg is unlocked at L5). IMP RFTs are useless.
Cluster all the turrets together. They are more deadly together than spreading apart. Turret destroying starships are only unlcked at L6, I think.
Most importantly, load SC with high level bikes and snipers. They’ll win 80% of your defense for you when your SC is full.