I understand the problem for the first one. The barrels actually prevent us to place turrets so that they cover correctly the base.
For the second one, we could also pack the base closer to the two doors so that the turrets aren't too spread out if needed.
What I like about that second base, is that humans get a lot of different ways out of the base and so aliens will have a much more difficult time pining the humans in their base, and two of the ways can be easily covered at the same time so that defenders don't need to spread their forces too much.