Philippi was a place of considerable importance during Paul's day. Here is a little background:
It was the sight of the historic battle where Octavian defeated Cassius and Brutus for the murder of Julius Caesar in 42 BC. In addition, Octavian settled a number of Roman veterans after the battle of Philippi in 42 BC, and again in 31 BC after the battle of Actium. As such, Philippi had the status of a Roman colony. In short, Philippi was in essence an extension of Rome. The city had the privilege of Roman law and most likely was exempt from taxes and tribute.
Judging from inscriptional evidence and archaeological remains, Philippi was an urban center of importance. It was the leading city of Macedonia. In the light of this, it is no wonder that Paul’s first foray into Macedonia and Europe was the city of Philippi. Paul’s missionary strategy was consistently urban. Acts 16 describes Paul’s missionary journey into this region, the conversion of Lydia, and his subsequent imprisonment and release.