Footprint

Footprint is the term given to the space taken up by a particular tower. The size of a tower's footprint is usually dependent on its power. Stronger towers usually have a larger footprint while the weaker towers usually have a smaller footprint.

Strategies
The size of the footprint can alter the strategies when trying to place strong towers on small pieces of land or water, where some towers cannot possibly rest on the part of the land or water.

In the Mobile and Steam games, turning on "Drop and Lock" in options will make it easier to place towers with different footprints. It will also help trying to align the tower as economically as possible for other towers to be placed, especially if there is tiny land/water space, such as on Pontoons or Portable Lakes.

In the Flash games, a common trick is to reduce the game quality to low. This slightly reduces tower footprints, allowing more towers to be placed.

On certain tracks, there is less space for towers to be placed, and thus making it harder towards the end game, due to most players typically adding Temples of the Monkey God with multiple sacrifices of large financial worth towards the end game. Tracks to be careful of include Space Truckin', Dockside, River Rapids, Cornfield, High Finance, and Peninsula.

Monkey footprints
From smallest to largest:
 * 1) All placeable sub-towers (consume 0 footprint)
 * 2) Dart Monkey, Tack Shooter, Ice Monkey, Glue Gunner, Sniper Monkey, Ninja Monkey, Alchemist, Banana Farmer, Tech Bot, Energizing Totem
 * 3) Boomerang Monkey, Bomb Shooter, Monkey Sub, Dartling Gunner, Wizard Monkey, Druid, Engineer, Heroes (except Churchill and Pat)
 * 4) Monkey Buccaneer, Spike Factory, Cave Monkey, Captain Churchill, Pat Fusty
 * 5) Monkey Ace (height)
 * 6) Mortar Monkey, Super Monkey, Monkey Village
 * 7) Heli Pilot
 * 8) Monkey Ace (width)
 * 9) Banana Farm, Pontoon(?)
 * 10) Portable Lake
 * 11) Sun Temple
 * 12) True Sun God

Note: Heli, Ace, Banana Farm, and Pontoon use rectangular placement. Weirdly, that seems to mean that, when one of them is involved in a placement, collision is determined by assuming BOTH towers are rectangular. For example, if you place an Ace near a Druid (which is not rectangular), the Druid will be assumed rectangular. Same for placing a Druid near an Ace. Sun Temple and True Sun God may also be rectangular.

Select sizes for stages

 * Hedge: There are 8 gaps to place towers, four on each side. The first four bends can fit: Spike, Spike, Boomerang, Dart. The last four are the same in reverse.
 * Cubism: Orange triangle near the end can fit a Dart. Orange triangle near center can fit up to Spike Factory. Small red triangle near center can fit Boomerang (but this requires intense precision).
 * Lotus Island: Bottom-right island can fit two Boomerangs.
 * Spillway: Barrel in the very center can fit a Dart.
 * Cargo: Edge of the highway can fit Darts.
 * X Factor: Center can fit a Mortar, or three Darts.
 * Off The Coast: All three boat pieces can hold Spike Factory.
 * High Finance: Peak of building right above the pond can hold Dart.
 * Dark Castle: Edge of wall can fit up to Boomerang.
 * Infernal: Upper-left and two side rocks can hold Heli. All rocks can hold Ace. Side rocks can fit Banana Farm.
 * #Ouch: Corners of puddle can hold Spike.
 * Carrier Flagship (tower): Up to Boomerang.

BTDB2
Rough estimates by exephur, a member of the r/btd6 community who also runs BTD6 Index:


 * All heroes are small size
 * Small
 * Dart, Tack, Ice, Sniper, Ninja, Alch
 * Medium
 * Boomer, Bomb, Glue, Sub, Wizard, Super, Druid, Engi
 * Large
 * Bucc, Dartling, Spact
 * Extra Large (Round)
 * Mortar, Village
 * Extra Large (Rectangle)
 * Ace
 * Extra Large (Square)
 * Heli
 * Extra-Extra Large (Square)
 * Farm

Ordered by tower type then volume.

Trivia

 * This is an official term as seen in the Bomb Shooter's Monkey Knowledge Card of Ranks 3, 8 and 13. It reads "Footprint reduced by 5%/10%/15%" respectively.
 * The Mortar Tower's footprint in BTD5 is the size of the barrel before the tower is placed. The tower can be moved at any position, and can be made to look as if it is on the track. Any more placements of any other Mortar Towers may be influenced by any other Mortar Tower barrels being already placed.