The Short Answer
Clear the Yellow Tower (the third tower in the recommended Red → Green → Yellow → Blue order). Completing it unlocks the Tunnels — the fastest movement system in the game — and also opens all the main-area coloured doors. The community guide explicitly calls the tunnels the quickest travel option, which is why Yellow is worth doing before your longest expeditions.
Why Yellow Tower
Each of the four towers grants a different travel unlock. Yellow's is the tunnel network, which beats the chairlift and monorail for raw speed across regions. Because the tunnels arrive at step three of the recommended route, you get the best travel tool right before the biggest late-game blorb hunts at Blue.
How Tunnels Fit the Movement Network
Tunnels are one part of a layered travel system, alongside:
- Train / Monorail — unlocked at Blue Tower (see start the train).
- Cable Car (Chairlift) — unlocked at Green Tower; the only route to the east-ridge challenge area.
- Shortcuts — easily missed bonus routes, often triggered by a stack or a tool.
Together they let you hop between regions instead of walking the whole way — though walking is very much the point of the game, so use them when efficiency matters, not every time.
Two details are still being documented by the community: the exact tunnel entrance locations and which regions each tunnel connects. This page reflects the verified unlock relationship (Yellow Tower → Tunnels). We'll add the entrance map as walkthroughs are published.