Many of those towers still survive, though none are as daunting as the great stone stockade on Arran, off Ireland's west coast.
And they didn't just spring up around the edges of the British islands， all over the mainland, too.
The great hill forts of the Iron Age remain visible in terraced contours at places like Danebury and Maiden Castle.
Lofty seats of power for the tribal chiefs, they were defended by rings of earthworks, timber palisades and ramparts.
Behind those daunting walls, this was not a world in panicky retreat.
The Iron Age Britain into which the Romans eventually crashed with such alarming force was a dynamic, expanding society.
From their workshops came the spectacular metalwork with which the elite decorated their bodies.
Armlets, pins and brooches and ornamental shields like this, the so-called Battersea Shield.
Or the astonishing stylised bronze horses, endearingly melancholy in expression, like so many Eeyores resigned to a bad day in battle.