Generally I wouldn't do that and within most canals I would never do it.

However, if there are heroes who could retrieve safely but won't even attempt to, and keep sending midbies, then I'll use orcs and tremblefist to address the issue.

Also, not all cabal guardians are equally difficult. The watcher, for example, is much much harder and more dangerous than the executioner, particularly if there's a defender.

And unwanted retrievers at the spire only have to flee one step to safety.

So I'm not sure why tremblefist should be the same as another outer.