I recorded two short videos of level 2 (patch 1.51 but exact same happens with 1.7) to show you my problem.
In the first video I claim the portal very early and play sort of like I normally would.
In the second video I build my rooms first before claiming the portal but then the bug where the game thinks I breached Lord Darius' walls happens out of nowhere (at minute 1:34).
Besides that I also tried level 2 with my 1.3 version of the game but it seems like that level is old compared to the 1.7 version because the path north of the portal is closed by walls (or maybe that's exactly why that one hero party invades my Dungeon?
First video
here
Second video
here