Hmm, yea, didn't think of that. I made the alternative routes because the main route is very easily to defend. But indeed, if humans rush immediatly and there would be no default tubes placed, there would be no way stopping them if the alien players ain't very good. I'm thinking about adding default tubes to stop an early rush. Once the aliens are s2, the path between the default will become useless for humans because the single stairway can be easily protected with trappers and stuff. The halls however, are more difficult to protect and easier to attack the default OM from a relatively safe distance, although doors will make it more difficult.
For aliens however, i think i'll add a trapdoor above the main human default base hall, because i think it will greatly unbalance the map if i would add a second entrance to the human base that allows big aliens to use. thx for the reply (whoa, this took me almost 15 minutes to write xD).
Edit: I think i won't add a second way for bigger aliens to enter the human base, instead, i connect the upper vents with the lower vents so that dretches can assault the human base from all sides. This way, the humans need to spread their turrets which will weaken the front, making it easier for bigger aliens to destroy the turrets upfront. Do you think this would help?