The soul of the deck, and the reason it works at all. King Drasil_7D6 has 3 effects, all that work from the breeding, and 1 inherited effect. 

First, you cannot digivolve while King Drasil_7D6 is on the breeding zone. That means you can only hard play Digimon.

Second, and this is the only optional effect, you can reduce the cost of Royal Knight by 4 plus 1 for each digivolution card under it.

Third, you add the top card of the Digi-Egg deck and all Royal Knights on the field under it. REMEMBER that this is on the Start of your Main Phase, meaning that even on turn 1 you will put another King Drasil_7D6 under the first one. That means that the first "discount" to play is 5 minimum. This is also an obligatory effect.

Fourth, on the inherited effect, is that when an Option card with the Royal Knight trait is PLACED in the battle are, you gain 1 memory. This means that the only options that trigger this (up to EX05) are Royal Knights of the Purge (BT13-110) and From Master to Disciple (ST12-15). Seiken Meppa does not place itself on the battle area, and thus, does not gain the memory.

Use 4. No need for other eggs.

The cheapest Royal Knight from BT13, and also one of the best opening plays if you are starting. King Drasil_7D6 (BT13-007) reduces the cost by 5 on turn 1, making this a 2 memory play. Also worth noting that this is one of the few ways this deck has (in archetype) to draw cards. You draw a card no matter how this card leaves play, including by King Drasil's effect.

Considering that this deck plays for the long game, having an early and cheap blocker that draws a card is very good against aggro decks, like Hunters, Xros and Red Hybrid.

On the late game, you normally don't want to use your reduction on this, and will resort to either use Leopardmon (BT13-056) effect to reduce the cost, or just hard play for 7.

Recommended 4. 

Besides running the show on his deck, Alphamon is also amazing by itself. The ability to stop play cost 10 or higher to attacking players for a turn shut down most of the single stack decks (BlackWarGreymon, WarGreymon, Alphamon, Beelzemon, etc). 

An important point about his first effect, is that it only affects cost 10 or higher Digimon from attacking the player, so it's still fair play to try and hit a Digimon that's suspended! On this same note, there's ways to bypass this like warping in the middle of an attack (like by using an X-Antibody, or Impmon (ST14-02) warp effect), as the Digimon that attacked initially wasn't a cost 10 or higher!

His built-in protection also helps a lot, since you can keep him around for a turn even under King Drasil_7D6 (BT13-007) effect, and hit the opponent. However, there is always the chance it will be deleted on the security battle, so choose wisely.

Worth noting that Omekamon (BT13-093)(we'll discuss it later) has the X-Antibody trait, and can be used with Alphamon to trigger the <On Play>.

Sometimes, it's better to not hold Alphamon on the field, and let King Drasil_7D6 take him, in order to use Gankoomon (BT13-019) to trigger its effect again.

Recommended 2-3. Depends on how much it affects your meta.

Being unaffected by Digimon effects for a turn is very useful on some match-ups (Blue Flare, ShineGreymon, etc). His secondary effect is often forgotten, and can help you remove some Digimon that are harder to deal with on this deck. An important distinction is that his second effect is optional, so you're not forced to make an opponent's Digimon attack. Also, only Digimon with Rush, or that were on board previously can be forced to attack.

It's worth noting that Belphemon: Sleep Mode (BT13-088) effect targets the attack, so it can still stop Craniamon attack.

Besides those match-ups, you don't want to see this card much.

Recommend 2-3.

The best searcher of the deck. Looking at the top four card of your deck, and choosing two cards with the Royal Knight trait helps a lot to grab the best Royal Knight for the situation and digs for the Omnimon. This also can take the Option cards, and that is really helpful.

The second effect is often overlooked, but it can destroy ALL level 4 our lower Digimon EACH time you play another Royal Knight on the field. You can use this in a pinch (with Royal Knights of the Purge of course) to remove floodgates or just clear the opponents board.

Recommend 4. ALWAYS.

One of the two secret rare cards of the deck, the reason this deck is so expensive. And it is worth the price tag. Gallantmon has its own reduction effect, that CAN be used in addition with King Drasil_7D6 (BT13-007) effect. Then, it has a single effect, that can be triggered on multiple ways. Delete a 6000 DP or less, or if it is not possible, a Digimon with 13000 DP or more. Nowadays, this misses the very important middle gap of 7000-12000 DP. But this is compensated with the fact that it has Rush. You will often put Gallantmon on play, delete one small Digimon, attack another tapped Digimon, and probably remove another Digimon. Triple removal for one card, not bad right?

Very useful on the mirror match.

Recommend 2-3. Depends a lot on how much the gap (7000-12000 DP) is on the meta.

The Joker of the deck. By itself, he doesn't do a whole lot. However, he is a N-th copy of all other Royal Knights. This makes both Jesmon (BT13-017) and UlforceVeedramon (BT13-030) way more effective. And being an13k DP blocker is just the icing on the cake.

Also, you can grab any Sistermon from the trash. This is important on the late game, since it will gain the Rush from Omnimon (BT13-112), giving you another attacker.

Recommend 2-3. Very useful with other Royal Knights, not by itself.

The small fry killer (most of the time), being able to remove any number of Digimon up to 6000 DP is good against go wide aggro decks (minus Hunters). It's worth noting that this effect increases by 2000 DP for ANY other Digimon, not just Royal Knights. Sometimes, it is better to not attack with some of the small Digimon on the deck to increase the DP you can delete.

Also, the second effect makes your Royal Knights (and Sistermons) very buff, making your blockers more efficient and overall less likely to die on security battle.

Recommend 2-3. Depends a lot on the meta as well.

Kentaurosmon has a very interesting effect, that allows for a more yellow control-ish build to work. Imagine inserting an Lonkhe Adistakto (BT10-101) on the top of your security? Amazing for sure, but the main reason we use this is for the 3 memory gain. This allows you to play Omnimon (BT13-112) passing the turn, and gain the memory back in order to attack on the same turn.

It also has utility of giving -7000 DP and unsuspending, being a really good target for the Royal Knights of the Purge (BT13-110).

Recommend 1-3. Just one on the regular builds, and up to three on more dedicated builds.

The only Level 6 Royal Knight from BT13 without an <On Play> effect. However, it compensates with the amazing <Main> effect. Reducing the play cost of any green Digimon or Royal Knight by 4 is really good, and giving all of the above blocker until end of the opponent's turn is just the icing on the cake. A great accelerator for playing Royal Knights on the field. Special mention to Salamon (BT13-048), that searchers for Royal Knights. Using Leopardmon effect, you can play Salamon for free, get a search, and have both gain Blocker is a incredible feeling.

An important distinction is that this effect stacks with King Drasil_7D6 (BT13-007) effect, so it's a good combo to finish games.

Recommend 2-4. Personal taste really.

The Royal Knight you want to see when shit happens. Sometimes you only find Omnimon (BT13-112) on the security, and this swoops in for the save. Recovering any Royal Knight trait card (including the options) is very clutch at times. The secondary effect is amazing when you have a lot of Royal Knights on the field, and was not able to win on the spot. Even more with a Craniamon (BT13-077) on the field together.

Recommend 1-2. Just in case.

One of my favorite cards on this deck, UlforceVeedramon is often overlooked. Its effect trigger even when you play by itself. MEANING, when you play UlforceVeedramon, you get to trash the top two digivolution cards of the opponent's Digimon for EACH Royal Knight and Blue Tamer you control. Then you ALSO get to return an opponent's Digimon without digivolution cards to the hand. Even if you only have UlforceVeedramon on the field. YEAH, really. Check the Q&A if you doubt me. Sometimes, not destroying a Digimon is the best way to handle some decks, and this card performs very well for that.

Recommend 2-3. Tested with some Matt Ishida (BT1-086) and liked it a lot. Just need less Psychemon (BT8-071) on the field.

Examon really shines on it's DNA Digivolution. Having that one extra attack sometimes makes the whole difference. The effect is really good, just a bit costly.

Also, being able to choose a Digimon to suspend, and not being able to unsuspend is really good against <Raid>, and even reusable with Gankoomon (BT13-019).

Recommend 0-1. I like having all Royal Knights from BT13 on the deck, and it has utility (and a different name).

The true boss of the deck, having two really useful effects. 

First is just "Gaia Force on legs", deleting any Digimon without any conditions. Do not be afraid to use this effect early on the game, sometimes you must expend your Omnimon to keep yourself alive or on a better board state. Remember, slowing down your opponent is priority number one. Also, it is very good to have one Omnimon under King Drasil_7D6 (BT13-007), since it will destroy another Digimon when you activate it's second effect.

Secondly, you can play one of each Royal Knight under King Drasil_7D6 without playing the cost. Then you trash your breeding area. And after all that it still gives Rush to ALL of your Digimon for the turn, including Digimon played after Omnimon (strange, I know, but I checked with the Judge Discord).

One thing of note, is that after you destroy King Drasil_7D6, you CAN digivolve. That means you can evolve Omnimon over Jesmon (BT13-017) / Gallantmon (BT13-111) / Gankoomon (BT13-019) / UlforceVeedramon (BT13-030) using 4 memory and still triggering the "Gaia Force".

Recommend 4. No less under any circumstances!!!!

Another low play cost Royal Knight, useful if your meta has a lot of early aggro with small Digimon. However, since Omnimon (BT13-112) checks the name of the Digimon, not the number, its not as useful as the example above. Some people like to max the "best" going first opening, and use this card.

Recommend 0-2. And only after you max the Magnamon (BT13-040).

The first of the four main techs for Royal Knights of the Purge (BT13-110). All of then does not have <On Play> effects. Good for removing floodgates/small Digimon, and if there is none, trashing a security card. Then, you can use the Raid to remove another Digimon. 

Recommend 0-1. Really depends on how much you draw Royal Knights of the Purge.

The second of the four main techs for Royal Knights of the Purge (BT13-110). All of then does not have <On Play> effects. You attack your opponent security once, suspend an opponent's Digimon, hope that you don't hit something that kills it (it normally does survive), then attack the suspended Digimon. 

Recommend 0-2. Really depends on how much you draw Royal Knights of the Purge.

The third of the four main techs for Royal Knights of the Purge (BT13-110). All of then does not have <On Play> effects. 

Jesmon GX work together with its Option card Seiken Meppa (BT10-110). You use the Royal Knights of the Purge (BT13-110) to bring Jesmon GX from under King Drasil_7D6 (BT13-007), attack once (hope to survive), use Seiken Meppa to unsuspend and trigger the <When Digivolving> of Jesmon GX. This allows you to put a Royal Knight from hand or trash and trigger its <When Digivolving> effect as well. Besides Magnamon (BT13-040), all Royal Knights have <When Digivolving>. And it does not matter if you pass the turn, since it also gets Blitz. So you attack again, this time with <Security Attack +1>. 

This play makes the deck kill your opponent faster, however, it adds a lot of "dead" combo cards, making the deck a lot more inconsistent.

Recommend 0-3. Really depends on how much you draw Royal Knights of the Purge. And should match the number of Seiken Meppa as well.

The fourth of the four main techs for Royal Knights of the Purge (BT13-110). All of then does not have <On Play> effects. 

Omnimon X Antibody is used for the same purposes of Examon (EX3-074). After purging, you can delete pretty much anything.

@marianoj on Discord put a nice comparison:

Examon advantages:

• Can check 2 if there’s nothing to delete (win a turn sooner),

• Can delete Digimon immune to deletion effects (Greymon, Alphamon),

• Can delete Digimon with evade (UlforceVeedramon, MirageGaogamon),

• Can DNA, granting two additional checks, or three if you play UlforceVeedramon (BT13-030) with it,

• Is an excellent a blocker if played in tandem or on top of Leopardmon (BT13-056).

Omnimon X Antibody advantages:

• Can delete Digimon with 15000 DP without dying,

• Does not have to survive or keep turn after checking a security in order to delete,

• Can delete an unsuspended Quartzmon (BT12-057) since it doesn’t need to attack twice,

• Does not trigger BlackWarGreymon (X Antibody) (BT11-074) effect,

• Is not hindered by blockers to delete what it wants,

• Can delete Digimon immune to deletion by battle (Garurumon, Seadramon)

Recommend 0-2. Really depends on how much you draw Royal Knights of the Purge.