Unfortunately you got pinched, and in pve that happens atm. The only real defense to fight this is to claim one tower beyond what area you intend to use to prevent this type of encroaching and just assume everyone will try and land grab your area. In this instance you have to assume the moniker of the bad guy land grabber a little bit because good guys do finish last in this case. But, in the end that only works if you do it before anyone pinches you. Only thing you can do now it hope they eventually forget about that tower or talk to them directly and ask them to move their tower or do a tower swap with them in an area that wont pinch your base.
