Yes. I see. It's important to avoid slide by accident.
Then you could add a two tap steps:
1. First tap would highlight the slot (maybe an overlay arrow indicating the direction)
2. Second tap would commit the action
By the way, can we expect a new version still this year?
////Edited: Tap slot could be a single tap. The game board is well done and a single tap would just make things faster and easier to play.
If you are afraid that some users would not like that you can make an option to disable this sliding method.
Anyway, I'm sure that a single tap (considering all the puzzle games I've played so far, since my Palm III) is the way to go.
