thank you all for the suggestions, I'll try them. morhping into a sprite also costy in 3dgs, considering fast and flexible camera movements, but maybe I can solve it for vegetation and units too. from Fragmotion it is also possible to export animated sprites...

@Superku: I don't fully understand how to apply a 2d sphere-sphere collision, but you are right, it woud be enough, even if units are moving on complex 3d buildings. and yes, my code also needs some performance optimization (I've already done a lot in this area to minimalize pathfinding costs). at the moment I cannot see how to solve to avoid the usage of while loops for each unit, but it seems to be a walkable way too.