In the late third century Saxon pirates were harrying settlements on both sides of the English Channel. If they came into the Exe Estuary they found that the Romans had organised a system of counter attack, a rapid response. Invaders coming from the sea could be spotted from High Peak above Sidmouth and from there the signal was passed , perhaps relayed by Woodbury Beacon, to alert the Roman Signal Station on Stoke Hill two miles to the north of Exeter . From Stoke Hill it is likely that messengers were sent galloping into Exeter and, by signal, the Roman port at Topsham was alerted directly, men shaken from their sleep, ships packed with soldiers sent away down the channels of the Estuary, by day or by night, to meet the foe.
Imagine those day and night watches, guards posted, called out, changed, flogged if they fell asleep. Imagine too those scrambled settings forth from Topsham, oarsmen pulling hard against the tide, soldiers packed on deck and wondering what kind of wild barbarians they had to deal with this time. Imagine the clashes in mid channel, the boarding parties, men dead and dying, swords clashing, blood on the mud.
I'm glad there's not too much of that sort of thing on the Estuary these days.