Luckily this map is only used foe this one encounter ever. I think you should relax and not worry about it. Cat is on the grass area which is about the same level as where the rothe is now. The Rothe is currently headed down a gentle slope and if it continued East 30 ft, it would be 1.5m down or so. and then wouldn't be able to charge Cat.
>We assumed it can't get on the green arc because that's raised
In that case, through the power of imagination, it is now raised just enough or the rothe is down just enough that it can't charge you currently.
>The idea was to lure the Rothe back to the driver and into Yulya's spell range while being on an elevated position on the green arc where the Rothe can see but not reach Cat.
I think it's fair. Let's just say the green arc is raised just enough that the rothe can't get onto it. But you will be arrested immediately for walking on the grass (just kidding.)
>Also I do not understand how diagonal movement works and how much it costs.
According to DMG, diagonal movement say of a 5x5ft grid would be 5ft-10ft-5dt-10ft etc. In this case we see a 10ft grid so it cost the rothe 10-20-10-20 to move diagonally so as you can see it moved roughly 4 diagonal squares. In medieval times, they didn't have digital calculators and paper was expensive so they estimated a lot.
