Crosspathing/Engineer Monkey

In Bloons TD 6, it is important to choose the best crosspathing for a specific tower in order to receive the best benefits out of that tower type. Towers have special roles to play for each stage of the game. The Engineer Monkey in its core is to produce various contraptions in order to assist the defense.

Introduction
The Engineer Monkey's earlier upgrades are designed to be a supporting role in the early-game, each fulfilling a different aspect that is not normally found with other towers. The key weaponry of the Engineer is sentries, which typically fill most of the DPS of an early-game Engineer. Its Tier 3 upgrades each provide a different later early-game role. Its later upgrades, specifically starting at Tier 4, are designed for a more defined role, Path 1 providing offense, Path 2 with support, and Path 3 with instantly destroying passing bloons. Crosspathing isn't important (with very few exceptions) for Engineer since the strength of the Engineer doesn't come its main attacks.

Tier 2 and below

 * 0-0-0: It has more pierce and attack speed than an unupgraded Dart Monkey, but it is slightly over twice as expensive. On some maps, even just one 0-0-0 Engineer can solo.
 * 1-0-0: Cheap but effective source of extra pops.
 * 0-1-0: Not very useful outside of sniping more bloons from further.
 * 0-0-1: Pops many more bloons per shot and can pop Frozen Bloons. Can be used for early game cleanup
 * 2-0-0: Further improves the sentry attack.
 * 0-2-0: Usually a stepping stone for Cleansing Foam.
 * 0-0-2: Stopping bloons with Pin is a useful early-game utility. This is the minimum for pinning a bloon in place, but adding further crosspathing beyond upgrading further to Double Gun won't improve its pinning efficiency further.
 * 1-1-0: Depending on the order this crosspath was purchased, the sentries may be in a focused area or more spread out.
 * 1-0-1: Both the Engineer and the sentries gain extra pierce.
 * 0-1-1: TBA
 * 2-1-0: Same as 1-1-0 but spawns sentries faster.
 * 2-2-0: Strong early-game MOAB-class popping power. Upgrading further to Sprockets will make even more use out of this potential.
 * 1-2-0: Only matters if you are planning to focus on top path upgrades.
 * 0-2-1: Not the best crosspath since its main attack doesn't deal a lot of damage against MOAB class bloons.
 * 1-0-2: Doesn't give the pinning effect until 3-0-2 which is why this is usually a stepping stone.
 * 2-0-2: The Engineer and sentries now get both benefits of the bottom path. Extra pierce and the pinning effect. 3-0-2 is needed for the sentries to pin, however.
 * 0-1-2: Allows the Engineer to greatly increase the usage of the pinning effect.
 * 0-2-2: Same as 0-2-1.

Sprockets

 * 3-0-0: The non-crosspathed Sprockets can be spammed for mostly maximum numbers of hits. Crosspathing will be quite useful to make the most of Sprockets, mainly mid-game potential and anti-MOAB popping.
 * 3-1-0: Extra range for the Engineer himself will make sentries go further from the Engineer if middle path was purchased first. However, if top path was purchased first then the player doesn't have to worry about the sentries spawning in awkward positions.
 * 3-2-0: While 3-1-0 causes sentries to be moved further from the Engineer and less consistently placed if middle path was purchased first. However if top path was purchased first then the added range no longer becomes an issue.
 * 3-0-1: Sentries gain more pierce and so does the Engineer. The engineer and his sentries gain more attack speed with Sprockets, so even more value packed into this upgrade.
 * 3-0-2: Sentries can also pin bloons with their attacks. Generally good against bloons that spawns more than 2 children

Sentry Expert

 * 4-0-0: The Sentry Expert, regardless of crosspathing, has special functionalities that other Engineers aren't sufficient alone. Its Crushing, Boom, Cold, and Energy Sentries all do special tasks dedicated for specialization at different situations. The majority of these sentries attack slower but all have quite high pierce and overall possess high damage and/or utility. Crosspathing for these sentries helps slightly for Sentry Expert but not significantly outside the mid-game where it tends to shine the best.
 * 4-1-0: Sentries already gain a sizeable amount of range already, the range buff for the sentries is insignificant, and the increased Engineer range is often worse due to how the Engineer may deploy sentries in awkwardly distant locations and in less focused areas. However, this no longer becomes an issue if top path was purchased first.
 * 4-2-0: Not as useful as 3-2-0 at single-targets due to the reduced synergy with Deconstruction, but better against grouped targets due to the much greater pierce of each sentry. That being said, many of these sentries aren't effective at MOAB-class anyway, even the Energy Sentries which are the best Sentry Expert sentry candidate for beating MOAB-class. Extra Fortified damage can be useful for handling mid-game Fortified Bloons.
 * 4-0-1: More pierce for each sentry, useful for handling dense mid-game rounds. Pierce bonus for these sentries is always roughly +25%.
 * 4-0-2: Mostly the best crosspath for a practical Sentry Expert, as Sentry Experts in general are better as a mid-game support DPS option. Cleaning up weaker bloons with Sentry Expert pairs well with the Pin effect to its sentries, especially due to the high pierce of all their sentries.

Sentry Champion

 * 5-0-0: The non-crosspathed Sentry Champion is somewhat weaker than a crosspathed Sentry Champion, but not as significant as some of the other Tier 5 towers in the game. Crosspathing is recommended to make the most of the massive DPS in exchange for the Sentry Expert's utility and multiple specializations.
 * 5-1-0: Extra range can be helpful for the sentries, although the wider range of the Engineer himself can cause sentries to move further from his range, sometimes awkwardly. However, the 5-2-0 crosspath will outweigh this slight overall negative effect from 5-1-0. This doesn't become an issue if top path was purchased first.
 * 5-2-0: This is usually the best crosspath for Sentry Champion, giving +1 MOAB-class and +1 Fortified damage per sentry. Combined with the Champion Sentries' super fast attack speed and decent damage and pierce, this equates to a lot more DPS during the late-game where MOAB-class and Fortified dominate the late-game. This certainly also helps allow Champion Sentries to beat Fortified Super Ceramics more easily too.
 * 5-0-1: Gains +1 pierce, which is a bit too small for Champion Sentry's 5 pierce. Ignore the main Engineer's nail gun attack, as it is the Champion Sentries' main attacks that do the most damage.
 * 5-0-2: The Pin upgrade helps the mid-game become easier to counter, pinning any bloon below Ceramic especially Regrows in place with ease with all Champion Sentries. Beyond Round 80, the Pin upgrade is mostly a vestige, as Pin cannot affect Ceramics or above.

Cleansing Foam

 * 0-3-0: For purely the Cleansing Foam effect, 0-3-0 is sufficient. Crosspathing is not essential, but either crosspath will improve its effect to different extents.
 * 1-3-0: Does not improve Cleansing Foam, but adds sentries that come with the Deconstruction bonus damage stats. Upgrade further to improve Cleansing Foam.
 * 2-3-0: In addition to faster sentry production, it produces Cleansing Foam quicker too.
 * 0-3-1: Cleansing Foams gain +5 more pierce.
 * 0-3-2: Does not affect Cleansing Foams, but adding Pin allows this Engineer to stop bloons with the Pin effect.

Overclock

 * 0-4-0: For purely the Overclock ability, this crosspath is sufficient. Crosspathing changes nothing to the ability. Overclock gains extra nail gun pierce too, making the 0-4-2 crosspath actually feel more significant for its main attacks.
 * 1-4-0: Sentries from an Overclock Engineer do not receive any benefits compared to a Cleansing Foam Engineer. Otherwise, treat these extra sentries as if the Engineer was 1-3-0.
 * 2-4-0: Sentries from an Overclock Engineer do not receive any benefits compared to a Cleansing Foam Engineer, and neither does the Cleansing Foam attack. Otherwise, treat these faster produced sentries and faster Foams as if the Engineer was 2-3-0.
 * 0-4-1: Just like 0-3-1 Cleansing Foam, but the nail gun's pierce bonus is insignificant.
 * 0-4-2: Takes advantage of the much greater nail gun pierce and therefore the Pin effect can be optimized with this crosspath.

Ultraboost

 * 0-5-0: For purely the Ultraboost ability, this crosspath is sufficient. Crosspathing changes nothing to the ability. Ultraboost gains even more nail gun pierce, but compared to its extraordinarily high cost it is insignificant.
 * 1-5-0: Same principles as 1-4-0.
 * 2-5-0: Same principles as 2-4-0.
 * 0-5-1: Basically takes principles from the 0-4-1 Overclock.
 * 0-5-2: Basically takes principles from the 0-4-2 Overclock.

Double Gun

 * 0-0-3: Double Gun can hold its own in the late early-game till mid game pretty well however, crosspathing should focus on applying the Pin effect as much as possible.
 * 1-0-3: Sentries produced from Double Gun are statistically equivalent to 1-0-1 Engineer sentries. Despite having the Pin upgrade, sentries do not pin unless given Sprockets or above. Merely, these sentries add extra popping power.
 * 2-0-3: Adds the maximum extra popping power for Double Gun by producing more sentries per second.
 * 0-1-3: This allows Double Gun to apply the Pin effect to bloons that are much further away from them.
 * 0-2-3: Generally a waste of money due to the fact that its main attack isn't made for popping M.O.A.B. Class bloons.

Bloon Trap

 * 0-0-4: This upgrade summons a trap that sucks in only bloons and not blimps. Once full, the player can hover on the trap to get some money. Since crosspaths make little difference for the traps, with a major exception to 2-0-4 and 0-1-4, crosspathing isn't the most important aspect of Bloon Trap.
 * 1-0-4: Does nothing except add sentries. Can be wasteful because fewer bloons go into the trap and therefore empty less soon.
 * 2-0-4: Even though it allows traps to be deployed much faster, this upgrade is generally not preferred because it tends to do the opposite of what the Bloon Trap wants, which is to let bloons go into the trap, but the sentries that spawns prevent some bloons from getting into the trap and therefore make less money overall.
 * 0-1-4: Since 29.0, this upgrade has generally become the preferred crosspath, as it grants the Engineer extreme flexibility thanks to the trap being able to be placed anywhere with its range. Considering that range is also extremely large, this means the Engineer can be significantly far away from the trap and still reap the benefits.
 * 0-2-4: Doesn't really help since its main attack isn't very good at taking down Blimps anyways, though breaking down fortified ceramics so as not to consume as much RBE can be useful in some situations.

XXXL Trap

 * 0-0-5: The only difference here is that it can now suck in all bloons that are not a B.A.D.
 * 1-0-5: Same principles as 1-0-4
 * 2-0-5: Reduces the cooldown between trap placements. Sentries don't backfire as badly since XXXL Trap's main prey consists of ceramics or stronger, which sentries are ineffective against.
 * 0-1-5: Same principles as 0-1-4
 * 0-2-5: Even worse as a crosspath compared to 0-2-4 seeing as the extreme capacity and MOAB absorption capabilities mean that its primary benefits are rendered completely irrelevant.

Strategy
TBA