I've been thoroughly enjoying the update and DLC. The Undercity mechanic for the Skaven is so much fun. I established an initial Undercity underneath Talabheim and, using the "Subterrainian Pit" building line it's been spreading outwards to neighbouring regions including Altdorf, Nuln, Wurtbad and Marienburg, generating income based off the cities own income per turn.
The discoverability mechanic is different from how I thought it was. I though it was a percentage each turn to be discovered, however, it's actually a score out of 100.
Say, you make a building that raises discoverability by 80 points. The enemy then builds a counter building in their city on the surface that increases their discoverability by +30. The 100 discoverability max is then reduced to 70, exposing your Undercity. As a counter, you can build your own counter buildings that lower your discoverability by -20 to -40 reducing your 80 to 60 or 40 and keeping it under the new threshold.
There's also a building that reduces your discoverability by -40 in one step but you lose vision over the region.
A nice detail is, as long as the settlement isn't razed, your undercity remains if the settlement above exchanges hands.
It's so much fun seeing the Skaven symbols floating over enemy cities and having them generate income for you all sneakily-like.