I wasn't originally going to tackle this idea, as I thought it was too much for 72 hours. I think I've stripped it down to something possible now.

Your goal is to deliver an important message from A to B. You control the messenger and a small number of units. Your challenge is an overwhelming enemy force trying to kill the messenger.

I thought it was going to be too much due to A* pathfinding, which I've never implemented before. Then I realised I could dumb it down to simply "move directly to the target position".

So far, I've got selecting and moving individual units working. I just had to fix a bug where I was selecting the first unit below the mouse on the Y axis, not the unit at the mouse position.