Navmesh Obstacle+Dynamic Pathfinding

here is a screenshot of my issue:

The screenshot says it all but basically how would i do a check for the last tower being placed down by the mouse and make sure the path does not get completely cut in half?

u can check for path separately of navmesh

and seems there is no need for navmeshm as your field already divided to cells and u can easely find path without navmesh