This story is a fable set in a magic forest full of intelligent animals that live in a huge community. Maleficent depicts a forest that's very close to what I'm thinking.
The magic present in the forest allows for the growth of a myriad of exotic plants, roots, seeds, etc. These items are of great value to humans, since they can be crafted into things like medicine, spices and garments (to name a few). This magic is also what made it possible for the animals to develop an intellect.
The humans never took residence in the forest - they used to come and go for what they needed. One day they did a very bad thing and were prohibited from ever entering the forest again. This ban happened a long time ago.
They live in huge cities where they enslave and kill animals (this is how the animals see farms). They can also build iron weaponry (medieval grade tech). Though there is no conflict or war going on, there are still those bold humans who think they can sneak into the forest and take the valuable stuff. There are constant attempts by individual humans to enter this forest.
To take care of that, some of the animals are responsible for keeping the forest's boundaries safe.
Suppose a group of "mercenaries" enters this forest after the goods. They're armed with medieval weapons (swords, lances, crossbows, heavy armor, traps, nets, etc) and they're experienced in fighting in groups against animals (meaning none of them will shit their pants at the sight of a lion).
From a tactic point of view, could intelligent animals defend against such attackers?
Things to Consider:
- Though the groups are heavily armed, they're not great in numbers. I'm setting the maximum number to 15 invaders;
- The forest is huge. There's no way that every entry point is heavily guarded by large animals, but there are hundreds of scouts spread across the border;
- Animals allowed are strictly those whose habitat is a forest;
- Neither animals nor humans are allowed to use magic. Both groups rely only on their physical capabilities, their tools and their intellect;
- Yes, animals can build tools - monkeys do have opposing thumbs. But their level of technology is lower than the humans'. They can only use whatever raw materials are available in the forest (no magic is allowed here either);
- The humans know that the animals that live in this forest are intelligent;
- Animals don't necessarily need to kill the invaders, making them retreat is enough - though that's not always gonna be an option, since the humans will sometimes be out for blood.
- As pointed by craq, The humans could start fires. There's a plot reason as to why they can't do it. You might accept this reason or the excuse that they won't burn the very thing they came for. But fires are out;
- As pointed by Nuclear Wang, I'm setting the minimum time for the humans to achieve their goal, assuming they're completely undisturbed, to 12 hours.