1) 치밀해! = I am not sure exactly of the context, but if it's an observation, "it's elaborate, delicate, carefully done, clever, etc."
2) 알 턱있나 = How can it possibly be known? (How can I know?)
3) 지켜줘야 되는 건가요? = Is it that I have to watch over it/you?
4_ 사람들 뭔가 비어보이지 않을까? = I wonder how it can be that people aren't visible (that nobody is here)?