why was the curfew in Constantinople?
The citizens of this town (i.e. Constantinople) are wiser and wealthier than aforesaid people (Persians, Armenians etc.). In order not to be atacked by neighbouring people every night they used to put their armed soldiers where 2, 3 or 4 streets meet so that to protect their city. After dark if the guardsmen find out that someone goes somewhere they catch him at once, birch him and for security`s sake they put him into irons and throw into prison; there they guard him till next day and then bring him to people (sic! – I don`t know what does it mean). Thus the city is watchfully guarded not only from enemies but from brigands as well.
Liutprand of Cremona, Antapodosis, I, 11