Advanced map

Mushroom Grotto

Info

Mushroom Grotto is an Advanced map added in update 54.0, the update that also brought the Root of All Nature paragon and the Tech Bot Prime and Hype Boost Monkey powers. It was designed by a fan in the Map Editor and was one of three winners of the 7th anniversary map competition, together with Ship Capture and Tricky Tracks.

Layout and path. Two lanes run at the same time. Bloons enter from the top-left and bottom-right corners and wind towards the opposite corners, with the two tracks crossing near the centre of the map.

The darkness mechanic. Most of the map is covered in spore-filled darkness. In the dark:

You cannot place towers.
Towers cannot see or target bloons, similar to camo bloons for a tower without camo detection.
Projectiles that are already flying still hit bloons normally.

Only the giant central mushroom gives light at the start, so you begin with a small lit area around the middle, where the lanes cross.

Lighting the mushrooms. Four smaller mushroom patches can be lit for $250 each, revealing more of the map. Once all four are lit, the large mushroom can be activated for $400 to enlarge every light radius. Lighting everything costs $1,400. You do not have to light anything: some towers actually prefer the darkness.

Workarounds. Mortar Monkeys and Dartling Gunners fire at a chosen point without needing a visible target, so they work in the dark from round 1. A Heli Pilot with Downdraft can still hit bloons it has just blown back.

Water: none. High ground: none. Line of sight: the darkness acts as the blocker.

The early game is the hardest part. From around the 20s, once you can afford the mushrooms, the map becomes much more like a normal two-lane Advanced map with a shared crossing in the middle.

How to beat Mushroom Grotto

Map overview for strategy

Mushroom Grotto is all about what your towers can see. At the start, only the area around the central mushroom is lit, which is luckily where both lanes cross. Everything you build early has to live in that circle and shoot bloons inside it.

Two ways to approach the map:

Light it up. Buy the four small mushrooms ($250 each) as soon as your economy allows, then the big one ($400). Your towers get more space and longer stretches of visible track.
Stay dark. Keep only the centre lit and use towers that benefit from a small visible area, such as Carpet of Spikes, which then drops every spike on the central section.

Whichever you choose, the intersection in the middle is your kill zone. Bloons from both entrances pass through it, so towers there effectively cover both lanes.

Best towers

Mortar Monkey: fires at a target point and does not need to see bloons, so it is effective from round 1. Aim it at the central crossing. 2-3-0 Heavy Shells, then 2-4-0 Artillery Battery or 2-5-0 Pop and Awe later.
Dartling Gunner: also fires without needing a visible target. A 0-2-3 Buckshot or 2-0-4 BADS aimed at the centre is strong.
Spike Factory 2-5-0 (Carpet of Spikes): with no small mushrooms lit, all of its spikes land in the middle section. Its huge single-target damage handles the BAD and makes non-dense rounds easy.
Tack Shooter 2-0-4 (Overdrive): at the central crossing where both lanes pass, it hits bloons from both directions.
Monkey Village 2-2-0: inside the central light, buffing your crossing defence.
Heli Pilot with Downdraft (0-3-x): can still hit bloons it has blown back, even in the dark.

Best heroes

Sauda: placed at the central crossing she hits both lanes and does not care about the darkness elsewhere.
Striker Jones: boosts Mortars and Bomb Shooters, the towers that work best here early.
Geraldo: flexible shop items that can be placed in the lit middle when you need extra damage.

Heroes that rely on long-range targeting are weaker until you light more of the map.

Easy and Medium

Put your hero and a Dart Monkey in the lit centre, then add a Mortar aimed at the crossing. Light one or two small mushrooms when you need room. Get camo before round 24 with a Village in the centre, and a 0-3-2 MOAB Mauler or upgraded Mortar for round 40.

Hard

Use Striker Jones with Mortars at the crossing, a Tack Shooter 2-0-4 in the middle and a Village 2-2-0. Light all four small mushrooms in the 20s and the big one when convenient. Finish with Pop and Awe or Carpet of Spikes.

CHIMPS

A realistic CHIMPS route that leans on targeting-free damage early:

Round 1: Sauda or Striker Jones in the lit centre next to the crossing, plus a Dart Monkey.
Rounds 2 to 15: a Mortar Monkey aimed at the crossing, upgraded towards 0-2-0, and a Tack Shooter in the centre towards 0-0-2.
Rounds 16 to 30: Village 0-2-0 in the centre for camo before round 24. Decide your plan: light the four small mushrooms for space, or keep the map dark for a Carpet of Spikes finish.
Rounds 31 to 50: Mortar to 2-3-0 Heavy Shells and Tack to 2-0-4 Overdrive. Village to 2-2-0. Round 40's MOAB passes through the crossing, where everything can hit it.
Rounds 51 to 70: Mortar to 2-4-0 Artillery Battery, Alchemist 3-0-1 on the Tack. Start a Spike Factory in the centre.
Rounds 71 to 90: build Carpet of Spikes 2-5-0, or Pop and Awe if you lit the map. Add an Ice Monkey 4-0-1 Embrittlement for DDT coverage.
Rounds 90 to 100: keep camo coverage at the crossing for DDTs. Carpet of Spikes' single-target damage handles round 100's BAD.

Impoppable

Light all mushrooms earlier for extra space, then follow the same crossing-focused plan with Perma-Spike or Carpet of Spikes as a safety net.

Dangerous rounds on this map

Rounds 1 to 20: the darkness limits placement and vision; leaks on the outer parts of the lanes are common.
Round 24: first camo, on top of the darkness.
Round 40: first MOAB; make sure your crossing has blimp damage.
Round 63: a ceramic rush on two lanes at once.
Rounds 90 to 100: DDTs and the BAD.

The BTD6 Master YouTube channel has video guides for Mushroom Grotto if you want to see the darkness mechanic handled live.