"Each tribe, when nomadic, may choose to move their tribe to any unoccupied intersection on the board - however, moving over edges marked with arrows may cost resources and gold.When a tribe is adjacent to a city, they may attempt to sack the city or conquer it - in either case, the number of horsemen or warriors (whichever is relevant) must exceed the city's defense value (which may vary from 2 for outlying cities and 5 for Rome itself).