For Amsterdam, the land was secured against
flooding, by means of dikes just before the
Amstel flows into the Ij (an inlet of the Ijselmeer).
As early as 1200, the first groups of houses
were erected on top of these dikes. In a second
stage of development, the Amstel was contained
by a dam (attested to in 1275) in the middle of
the settlement, and here the town hall and the
Nieuwe Kerk were built. The dam divided the
river’s course into a lower, open, harbor and an
upper, closed one. Ship traffic and all the
flowing water were diverted by two new canals
on the left and right of the Amstel. Later, two
more moats were dug for defensive purposes
and fed by water from the Amstel.