The tale of the TT Circuit at Assen is really the tale of Grand Prix motorcycle racing. That is hardly surprising, given that the race has featured on the calendar since Grand Prix racing was born, or rather, since the FIM Motorcycle Grand Prix Road Racing World Championship was established, back in 1949. And like Grand Prix racing, it has roots which go back a long way before that.
The first race took place in 1925, a year after the Dutch government passed a law permitting racing on public roads. It ran over cobbled roads and sand tracks between three villages to the east of Assen: Rolde, Borger, and Schoonlo. The next year it moved south of Assen, again over public roads, between De Haar, Oude Tol, Hooghalen, Laaghalen, and Laaghalerveen. It stayed there until 1955, when the first sections of what would become the modern circuit were built. The roads were closed and the circuit was separated from the world, an isolated loop of tarmac, where racing was safer, easier to organize, and, not coincidentally, easier to monetize.
The inaugural Grand Prix season in 1949 took place mainly on circuits set out using public roads, which made for long tracks taken at high speed (Bremgarten in Switzerland and Monza in Italy were the two purpose-built circuits on the calendar, but Bremgarten, in particular, was a spectacularly dangerous circuit which wound through a forest).