、、。
The shelling got more and more intense, so we ran down into the basement.
Literal
Shelling [topic-は] more-and-more intense-[became-くなってきた] [because-ので], we, running-[sequential-て] basement [into-に] descended-went [emphasis-のよ]
Lots of texture. ますます = 'more and more, increasingly'. ~くなってきた = 'has been becoming X', marking a gradual change approaching the speaker. The final のよ is a feminine ending combining explanatory の + よ — gives the sentence a conversational, narrating-to-a-friend feel, typical of feminine speech in older fiction. 砲撃 = artillery shelling, bombardment.