Thanks for the comments so far. Yes, indeed, we aim for a much faster pace than the first part (which feels really slow in comparison to me, now that I've picked it up once again).
Basically, we sold our souls to Unity3D. We started the first prototype I think three years ago, which had quite some iterations -- basically the game as it is now is almost completely different to what we had back then. Most things sounded good in theory, but just didn't survive "real users" when we sat them down and asked them to play. As harsh as it was, we killed a lot of our darlings in the process .
Back then, Unity had no decent 2D support at all, but luckily there is an asset called 2D Toolkit that's really great. During the development, Unity caught up and added 2D capabilities, but we never made the switch. Don't know if 2D Toolkit is still necessary nowadays, but the tilemap editor alone is great -- which is one of the reasons why we are using Unity3D, at all: the process of design is quite quick with rather short cycles. Just change something in the editor (or pause the game and tweak around a few values, then continue playing immediately).
I know, I'm drifting off now. To sum it up: we both had our share of "rolling our own engine", which is why we were looking for something more high-level that would still allow us to do fancy things if need be. A choice that I did not regret thus far.
We're still doing this mostly for fun, which is why we can be disgusting and offensive