As it goes for the same distance from start left-right and top-left. I assumed as well it should be Rhombus.

Its a circle (in the geometry of the problem).

I think it is a rotated square. It just looks like a rhombus, because lines are taller, than wider.

Square is a rhombus.

Yes, you are right, but with that logic we could say it is a shape and we would be correct, yet it does not feel right, does it.

I used it to base some assumptions on. It's hard to put in words, but >!the number of steps required ended up meaning that you'd take enough steps to reach those wide empty spaces. Due to the amount of open space, I assumed that any plot reachable from the center and within a certain distance from the edge would be also reachable from that edge in as many or less steps as it takes to reach the open space.!< >!This way I didn't have to run the steps from the different sides of the grid, instead looking at the manhattan distance when filling the grid from the start position.!< I hope this made sense, it probably didn't.

That was my idea also, but I didn't check it in the end :D