What was the most challenging part of development?

I'm also dealing with chasing character. Still didn't sort it out. However i'm trying to achieve so the playable character could come to the chasing character and have a dialog with the charater. It means to get them standing next to each other with the right distance and angle based on the current scene/way system.

This is a bit tricky, but definitley achievable. I've got my main and chasing characters throwing punches and kicks and knocking each other to the floor if the hit was in range.

For me the most tricky part of development is to create a full keyboard gameplay (instead of default mouse point&click).

But I also had a challenging time in Catyph to create the spaceship arcade minigame. It's a very simple game, but there were a lot of things to prepare carefully: asteroids collisions, countdown, game over or victory, score, etc.
You can try an old prototype here if you're curious: http://bit.ly/2jsSaxi

I also tried to include realtime 3D characters in Visionaire from 3dsmax, but I think that it doesn't work well. I really admire some of you here who decided to create games with this feature. Congrats guys!