 At this time Semaphore , with its jetty built in 1860 , was the state 's main entry point for passengers and mail . It had a signal station ( built 1872 ) and a time ball tower ( built 1875 ) . Semaphore remained of great maritime significance for the state until the 1880s . When the decision was made to build the fort , in 1878 , the state 's population had reached approximately 250 @,@ 000 . Settlement extended beyond Port Augusta , though Adelaide remained the dominant feature in the economy partly due to the layout of the rail network . At this point Adelaide had a population of over 30 @,@ 000 . Port Adelaide was the main port for South Australia , with over 1000 ships visiting each year , and a local population of over 2 @,@ 500 . 
