Does your pathfinding need to update when the terrain moves? If so, it will require a lot of computation to recalculate each body's path every frame. You could make it so whenever something hits a wall it regenerates the node graph and makes a new path.

How much micro-management do you want the player to have? Does the player build everything in the city, or could you have a governor (like in Civ) manage the city for the player according to a growth curve.

I do not understand why the impulse I apply to my object does not cancel its lateral velocity. I'm looking for an overview of my code, to help me see if I misplaced a variable. If code help belongs on StackOverflow only, I'll gladly ask it there.

I do not have a desired velocity. I have the heading an object is pointing, and it's current velocity. I could not just rotate the velocityvector to match the heading, that would not be physically accurate. ---- While the heading could be a vector, I think it is easier to keep it as an angle, then convert it to a normalized vector on demand. If I do not use angles, how would you suggest I rotate the heading of an object 45 degrees? How would I calculate how much force to apply to each axis as a result of engine thrust?

The heading is independent of the velocity. The entity could be pointing north, but moving north-east.

That was an interesting problem. I thought pygame would take floats (it does for blit, but not draw). What I did was put the int() around the newx inside the draw.circle method, so that while draw.circle received an int, newx was still a float. Have edited into my answer above.