You could call AstarPath.active.Scan () every time you change something, however that would be quite slow and might make your game lag.
Instead you could use an, right now, undocumented function called SetNodes.
For example you can call.
AstarPath.active.SetNodes (false,myTower.bounds,true);
This will set all nodes (if no colliders are there) in that area (myTower.bounds) to unwalkable (false, first parameter) and it will use FullPhysicsCheck (true, last parameter), which means it will check if the area is free from obstacles first instead of just setting the area to unwalkable whatever what. If you use FullPhysicsCheck, make sure that the tower isn’t instantiated yet, otherwise the script will find the tower and think that the area is still unwalkable.
You can find the full syntax in the AstarPath.cs script (haven’t written a page for it yet).

PS: Remember that you need to do a repath for all units when you have changed something, they don’t do that automatically.

Sorry if it’s somewhere on this site, but i didn’t find it anywhere!

Thanks!

When i grow up, and start selling games i will buy it!

i am moving 50 AIs and they are constantly updating their target (to follow a player)

As you mention the CharacterController is expensive. I replace the move function with simply giving the AI’s a rigidbody (i had to do this anyway) and pushing the around with physics (force) in their forward direction. (this ofcourse makes them ignore terrain height, but unity has a terrain height function to return the height of your location if standing on a terrain and i simply translate them up in the Y direction to match the height)

probably not the best solution, but it solved the problem for me with very little work

Thanks for your work by the way, saved some trouble

Are you trying to control 100 AI’s at the same time! Well, then you really need some speedups.
But, are you going to send a pathfinding call to all those 100 units at the same time, or are you giving them orders at different times?
First, the AIFollow script uses CharacterControllers, if you are going to use 100 units, I would not use CharacterControllers since they can be quite expensive.
Instead you can use Transform.Translate and use Raycasting for the height, works almost as good.
You can also see to it that all “direction.magnitude < something" statement can be replaced by "direction.sqrMagnitude < something*something" which will be much faster (square roots are not the fastest things to compute).

Are you seeing slowdowns when you send a Pathfinding call or is the performance slowdown the same the whole time?

Because you posted this question, I tried to make a scene with 100 units and tried to make it run fast, it resulted in the tips above (along with some next version stuff though)
I have posted it here: http://www.arongranberg.com/unity/a-pathfinding/massive/