。
I once spent a bit over a month in Nagoya.
Literal
1 month more-than Nagoya [location-に] existed thing [subject-が] exists.
~たことがある here with 居た (past of いる) marks past experience: 'have the experience of having been in Nagoya.' あまり after a time duration means 'more than, a bit over' — distinct from the negative-pairing あまり~ない ('not very'). 居る (居た) is a less-common written form of いる; most modern Japanese text writes this just as いた.