Comment 8 for bug 1534962

A rule of Widelands has always been that you cannot lose territory as long as you do not destroy you military buildings and as long as none of them are conquered. Neither of these two conditions is fulfilled when an enemy builds a port close to your border.
The correct approach would be to somehow send soldiers to the port space who will attack the military building (i.e. the tower in this case). See bug 1191296 for more information and a discussion about different possibilities. It will not happen for build19 though.