those are called Panzerkuppel or Panzerturm. generally obsolete or knocked out tanks had their turrets removed for use as fixed fortifications if there was a benefit. guarding airfields or for use on the Siegfried line. these were simply delaying measures. since the fortifications couldn't move, it was only a matter of time until they'd be knocked out by artillery or tanks or anti-tunk guns. you could also bypass them and deal with them later. there are still a handful of these around that have been left as WWII monuments.
![](http://i49.tinypic.com/erftah.jpg)
sometimes late in the war, during city defenses, a tank would be dug in and operate as a panzerturm. either due to tank damage, or lack of fuel, or desperateness of the situation. ![](http://i.imgur.com/BQWo4hF.jpg)