。
A storm was approaching our town.
Literal
Storm [subject-が] we [possessive-の] town [to-に] was-approaching.
近づく ('approach') is intransitive and takes に for the destination being approached — not を. This is a regular pattern for Japanese motion verbs that target a place (行く, 来る, 着く, 近づく). The ~ていた form makes it past progressive.