[Progress reports on funded work]

I really enjoy reading these posts.

Could the foot of the siege tower be non-axis-aligned?

The corners of the impassability fringe could be rounded; loci rather than just grown bounds?

[Progress reports on funded work]

I imagine you are maintaining a width*height passability map for the static obstacles? Possibly one per maximum-navigatable-incline if different unit types have different capabilities. Using a trivial neighbour-count you can easily find those those impassible cells that are islands; you can erase them from the passability map perhaps. You might actually store in each cell of the passability map the distance north, south, east and west to the next obstacle. Do keep posting results /Will (from Glest)