Hmmmm could be a tunnel to either the fortress that the monkey girl army is about to siege or a tunnel to the forest maybe, but i'm betting more in the first one, it should be the logical one, since they used the slaves in the fortess to make that earthwork fortification. The forest option could be a possibility, since they appeared from there like it was nothing, and it could be used to reinforce Shibashou's assault right now, but i don't think it should be this one. Welp, we will see in the next chapter maybe.
Anyways, congrats on the anniversary! It's beeing a big ass long road xD