I call this a bug... Is it?

[Ironhawk]

Oh, and as for the armies "switching places" thats not technically what happened. The order of events would have been as follows:

*Magic Phase: Teleporters gain control of castle prov
...
*Friendly Move Phase: Your army moves to the (now friendly) castle prov
*Attacking Move Phase: Enemy army moves into your (now vacant) prov

[Tuidjy]

I think that Ironhawk got it EXACTLY right. But you must agree
that it makes no sense, as the last step is basically:

The enemy army moves, unopposed, out of a besieged castle.