I actually wrote a little bit about fire arrows just a few days ago here. Like literally every historian I maintain that fire arrows are not meant for battlefield use. They are, as stated the, absolute last resort for a besieging force to set something on fire if you have nothing else available.

So what about a pitch ditches?

No. I have never ever heard of a pitch ditch used defensively. Why? Well... why would you ever do that? It would be immediately apparent to any attacker who could just throw a torch in it, wait for it to burn out and then step over it. Also you'd have to know almost exactly when an attack is going to come or whatever flammable substance you fill it with is just going to soak into the ground and even if you somehow managed to magically get tons and tons of flammable substance, undetected, into a trench just before an enemy attack and set it ablaze at just the right moment... then what? You'll burn at best a few dozen men at the cost of enormous quantities of flammable substance. A much more effective use of it would be to pour it from the walls already ablaze whenever someone tries to climb said walls or knock very forcefully on your gates.