Town and Fence

One English word which has interested me for some time is the English
word *town* which was derived from the Anglo-Saxon *tun* meaning a
settlement. This word is of course found in thousands of English
place-names such as Workington, Darlington, Southampton etc. I believe
that the word was used in this context from a very early period of
Anglo-Saxon settlement in what is now England. What puzzles me is that
there does not seem to a corresponding usage in north German, Dutch or
Frisian place-names, indeed the word appears to be related to the
German *Zaun* which means a fence. Did the term originally mean a fence
rather than a settlement and does it imply that the Anglo-Saxon
settlers surrounded their settlements with fences for defensive
purposes perhaps? Maybe it suggests a border or boundary (between
landowners ?). Are there any related words in other Germanic and indeed
Indo-European languages and if so are they ever used in the English
sence of settlement.
Thank you in advance.
David James