I don't know if a turret would help... it would maybe give us some breath in the early game, but by the time you get machine guns, the most dangerous monsters are bulletproof (and anyway, wouldn't the monsters attack the turret instead? especially the flying ones and the ones with a ranged attack?).
By the way, a problem that bothers me is covering the retreat, especially when the enemy has fast monsters, e.g. the Icefang and Magmatooth (the tigers).
I order the retreat when I think it's the best, and according to Murphy's law, the enemy spawns a Magmatooth that wipes away the slowest retreating units. For this prupose, I sometimes tried to sacrifice a fast Scarlet Wing: but it's very weak and the time gained is not so much... and even a "cheap" Scarlet Wing still costs a gene...
A similar "scaled down" situation occurs in the early game, if the enemy spawns a Scarlet Wing that bites my back during the retreat. And at the beginning, when I'm struggling to survive, I don't have genetics and so I can't spawn Scarlet Wings or any other fast unit.