イ号型局地迎撃天使NANAKO～かく戦えり、乙女達～

Humans have built a huge defensive city 3000 meters underground to save themselves from mysterious "vector" enemies. However, as resources get exhausted in constant battles, humankind throws last funds into new type of weapon IMA-01 Nanako. This weapon is given to main character as the last hope for survival.

[From vndbreview]