Advanced map
Underground
Daily challenges
More Advanced maps
Info
Underground is one of the four Advanced maps that shipped with the original release of BTD6 (version 1.0). It takes place in an old mine deep below the surface, with mine-cart rails, glowing blue crystals and bats hanging from the rocks.
Layout and path. Two lanes run at the same time along a pair of interconnected rail loops shaped a bit like a light bulb:
- One lane enters at the bottom left and exits at the bottom right.
- The other enters at the top right and exits at the top left.
The two rail loops cross each other in the middle of the map, so the lanes weave together rather than staying apart.
Lane length. Both lanes are very short, among the shortest of any Advanced map; only Erosion's lanes, Mesa's bottom lane and Cargo's MOAB lane are shorter. Bloons reach the exits quickly, which is the map's main difficulty.
Line of sight. The cave walls block line of sight only slightly, so most placements work as expected.
Placement. The centre of the map, plus the areas just above and below it, cover both lanes at once. These are the best spots on the map by far.
Water: none. Removable objects: none. High ground: none.
Bats. Tap a bat and it flies away, returning a little later. Purely cosmetic.
Despite its very short lanes and occasionally awkward targeting, Underground is close to an average Advanced map, because so many spots cover both lanes. It feels like a shorter cousin of Pat's Pond, but with a central space that sees everything instead of lanes that split apart.
How to beat Underground
Map overview for strategy
Underground is short and fast, but it is generous with good spots. The centre of the map, and the space directly above and below it, reaches both lanes, so most of your defence can sit in one compact cluster.
Key points:
- Build in the centre. Towers placed there cover both crossing rail loops.
- Stall everything. With lanes this short, slowing and stunning bloons is worth more than on most maps. Relentless Glue is a standout.
- Watch targeting. Two crossing lanes can make towers pick awkward targets. First or Strong targeting on your main damage, plus a cleanup tower near the exits, helps.
Best towers
- Boomerang Monkey 5-2-0 (Glaive Lord): its ricocheting glaives bounce between the two lanes for huge damage, and its orbiting glaives clean up both lanes when placed in the central space. Glaive Ricochet (3-2-0) and MOAR Glaives (4-2-0) are strong mid-game steps.
- Spike Factory 2-5-0 (Carpet of Spikes): every spike from its ability lands where bloons will actually pass, so it reaches full value every round. It cracks ZOMGs, beats DDTs and makes the BAD trivial. Pair it with a cleanup tower.
- Glue Gunner 5-x-x (The Bloon Solver): clears super ceramics easily and its puddles chain through lower-tier blimps. It pairs perfectly with Carpet of Spikes, which handles the ZOMGs, DDTs and BAD that Solver struggles with.
- Wizard Monkey 0-2-5 (Prince of Darkness): the crossing lanes give it plenty of zombie bloons to work with. A great mid-game that helps you save for more expensive towers.
- Glue Gunner 0-2-4 (Relentless Glue): stalls bloons on the short lanes and adds stun synergy.
- Monkey Village 2-2-0 and Alchemist 3-0-1: support the central cluster.
Best heroes
- Sauda: placed at the far left or far right of the central space, she beats the first few rounds alone and keeps solid coverage of both lanes all game. With stunning towers like Relentless Glue her damage skyrockets.
- Obyn Greenfoot: buffs Wizards for a Prince of Darkness build.
- Psi: global range catches anything that slips past the central cluster.
Easy and Medium
Put Sauda or a Dart Monkey in the centre, add a Boomerang Monkey and a Tack Shooter beside it. Get camo by round 24, and a 3-2-0 Glaive Ricochet or 0-3-2 MOAB Mauler for round 40. Finish with Glaive Lord.
Hard
Central cluster with Sauda, Glaive Ricochet into MOAR Glaives, a Village 2-2-0 and Relentless Glue. Add Glaive Lord around the 70s and Carpet of Spikes for the 90s.
CHIMPS
A Sauda and Glaive CHIMPS route from the central space:
- Round 1: Sauda at the left-most or right-most spot of the central space.
- Rounds 2 to 15: Boomerang Monkey in the centre going 2-0-0, then a Dart Monkey or Tack for extra early damage.
- Rounds 16 to 30: Village 0-2-0 for camo before round 24, then 2-2-0. Boomerang to 3-2-0 Glaive Ricochet.
- Rounds 31 to 50: Glue Gunner towards 0-2-4 Relentless Glue. Alchemist 3-0-1 on the Boomerang. Round 40 is handled by Glaive Ricochet and Sauda.
- Rounds 51 to 70: Boomerang to 4-2-0 MOAR Glaives. Optionally a Wizard towards Prince of Darkness for extra cleanup and DDT help.
- Rounds 71 to 90: Boomerang to 5-2-0 Glaive Lord. Start a Spike Factory towards Carpet of Spikes.
- Rounds 90 to 100: finish Carpet of Spikes 2-5-0 for DDTs, ZOMG shells and round 100's BAD. Keep Sauda's abilities and glue stuns for the toughest rounds.
Impoppable
The same central plan works. Get Relentless Glue earlier because short lanes punish any slowdown in your damage.
Other modes
In Half Cash, Sauda plus a single Boomerang line carries much longer than you might expect, so delay the Village until you truly need camo. In Double HP MOABs, blimps leave the short lanes very quickly; add a Maim MOAB Sniper or extra stuns before round 60 and get Carpet of Spikes earlier. In Alternate Bloons Rounds, the early MOAB-class bloons make Glaive Ricochet and Relentless Glue even more important.
Dangerous rounds on this map
- Round 24: first camo on two short lanes.
- Round 40: first MOAB, reaching the exit quickly.
- Round 63: a ceramic rush across both crossing lanes.
- Rounds 90 to 100: fast DDTs and the BAD. Carpet of Spikes is the usual answer.
The BTD6 Master YouTube channel has video guides for Underground if you want to see the central build.
Frequently asked questions
Is Underground hard?
It is close to average for an Advanced map. The lanes are very short, but the centre covers both of them.
Where should I place towers on Underground?
In the centre of the map and the spaces just above and below it, where towers reach both lanes.
What is the best hero for Underground?
Sauda, placed at the far left or right of the central space, especially with Relentless Glue.
How do you beat Underground CHIMPS?
Sauda and a Boomerang path to Glaive Lord in the centre, with a Village, Relentless Glue and Carpet of Spikes for the final rounds.
Why do bloons leak so fast on Underground?
Both lanes are among the shortest of any Advanced map, so slowing and stunning towers like Relentless Glue are important.
Does Underground have water or removable objects?
No. Underground has no water, no removable objects and no high ground.
Guides for every mode
Medium4/4
Hard5/5
Impoppable2/2
