ok i have been visiting the one ring forums for years but this is my first post

can a model defending an obstacle such as a wall or cliff face, be supported by a spear.

i can understand walls, but cliffs?

one of my opponents is a very cowardly moria goblin, and likes to sit on every hill he can. so climbing up ledges to fight is not good if he has 2 or three goblins each supported by a spear against one model climbing, who, if he does win may strike the cliff first

and can attacking models, against walls or cliffs be supported?

Firstly, welcome to the One Ring!
And to answer your question, yes you can support while defending a barrier

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great, thankyou, but what about attacking that barrier

or when climbing a cliff can you support the guy above you