Thanks very much for the detailed feedback!
I agree that using tiles that actually resembled more grids, or even highlighting the grid lines would've been better. I also wanted to add shadows to the characters to better show where they are but my Photoshop skills weren't high enough for the task. :D
As for goals: you are correct both on that you need to reach the stairs and that you need to get the characters together. :) On each level, there is a signpost and some shiny-flower-like things on the front of it that shows the only tile where you can approach the stairs from. The goal is to "Re-Join" the pair at the exit sign, so getting both characters on the same tiles anywhere in the middle will be accepted.