Interesting question. The most challenging was probably getting the character controller working, with the right feel, and working in all circumstances (slopes make character controllers 1000% harder!). The most time consuming possibly the graphics but not sure - might be about even with coding. Had to build several surrounding programmes too (map editor, Population editor (adding monsters etc), atlas image creator) which took a while (esp the map editor).
With more planning it would have been faster but hey, it's been an organic growth which has allowed testing for fun along the way.
The level design and building will take a loooong time next - That's a mammoth task. You can only really get started on that when you know what balance you need to achieve - number of coins etc - I've got a shop coded that I've not shown yet

Tonnes done in the background.
Using Tier 1 AppGameKit V2
Started coding with AMOS (Thanks Francois Lionet)