The Secret of Killimooin (Children's Book Classic). Enid blyton

Prince Paul, still attending the same boarding school as Jack and Mike Arnold, invites them and their sisters, Peggy and Nora, to his palace in Baronia for the holidays; but it is so hot there in the roasting summer that the entire royal household adjourns to their castle up in the mountains. Bands of robbers are marauding the countryside, terrorizing the local people with their savagery, and Paul is incensed, and determined to find the robbers and have them detained – and, together with his friends, sets out to do this. They discover a secret tunnel in a cave on the mountainside which leads to an underground river, which eventually flows into a secret forest which is entirely inaccessible from outside, owing to being entirely surrounded by immensely tall cliffs and mountains which are completely impassable. It turns out the robbers live here, and only they know the secret tunnel leading into the forest – which is why no-one has been able to detect them for years.