Impassable or not?

The recent AOS World Champs 2022# saw the event organisers add "impassable" terrain comp rule to the event. This article discusses some of the nuances and implications of adding "impassable" to your event.

Background & History

The 3D terrain rules within AOS have been somewhat problematic since day 1. Fundamentally, the game is not really designed for use of 3D terrain, and the rules team seems to have ultimately opted for a combination of short streamlined terrain rules and "let the players work it out via common sense". Of course, in a highly competitive setting, "common sense" can be a problem where there is advantage/ disadvantage to be had via using exact RAW rules!

This has historically seen terrain rules exploited - such as a deep-strike shooting unit landing on top of a 15" tall terrain piece and blasting away with impunity, becoming effectively impossible to reach. Or immersion breaking moments where some huge base flying monster hangs off the side of a tiny flagpole and then skateboards down a wall to slingshot past some front line screen-blocker unit. Various terrain exploit interactions can and have created contentious and even "feel bad" moments in the game.

Impassable terrain can add some really cool and interesting tactical elements to a game (eg chokepoints), and can also 'solve' some of the 3D terrain exploit rules above. However, while adding impassable can 'solve' one problem, it creates a plethora of other downstream problems.

To Impassable or Not

So, should you - a new to scene TO - add Impassable terrain or not to your event? After all, the World Championships# did it, so it cannot be that big a deal right?

To be really clear "impassable" is definitely NOT part of the core rules, so adding in Impassable is a TO override on core rules. The game is not designed or balanced with impassable terrain included. There are SIGNIFICANT downstream meta winners & losers implications for the for the game if you do add in impassable as a comp rule to your event.

Winners & Losers

The main impact of adding impassable terrain is it inhibits movement around the table. In short, this creates significant advantages for armies/ units/ playstyles that are desinged to benefit from movement being inhibited, and massive disadvantages for armies/ units that want clear space and nice clean run at the enemy.

Meta Winners:

1. Sevireth, Lord of the Broken Warscroll with his buddy foxes:

The LRL Fox + Sentinals list is already a powerful list with long term proven success. The lists core aim is to use the Foxes extremely annoying movement shenangians to bottle up the opponent and buy extra time for the Sentinals shooting to blast the opponent off the table. Adding impassable massively increases the Foxes control efficiency at choke points

The Fox+Sentinals list is also generally rated amongst the most frustrating and poor play experiences for opponent players. Frankly, adding in Impassable terrain improves the power of this already very strong list into "broken" level territory - and is basically an open invite for players to bring along this list and just walk onto the podium.

Another variant on this same theme is the new IDK Rune of Surging Gloomtide. The Rune summons new defensible* boats clogging up the board even further to buy more time for all the IDK shooting to do its work. Impassable terrain combined with the IDK Rune plus Sevireth falls into the 'game breaking' category.

2. Chaos Control Lists (eg LOFP, certain BoC, Tzeentch, nurgle or khorne builds)

The 'control' list playstyle is generally designed to win via controlling the board space and thus forcing opponents into inefficient fights - eg Archaon having no other option except to waste an entire turn chewing up some 70 point chaff ungor screen (after being locked down by Be'lakor for 2 turns of course!). The control player buys enough time by clogging the board and denying objective points and battle tactic points that they win the game, often dealing very little damage output.

Impassable aids this playstyle significantly, allowing multiple layers of cheap chaff screens to clog up choke points, and making it harder to bypass those screens.

Key units that specifically benefit via an increase in efficiency value include:

- STD Deamon Prince with Mark of Khorne

- Furies

- Kairos

- Sloppity Bilepiper

- All super cheap chaff screen units (eg ungors, untamed beasts)

- Be'lakor

The efficiency value increase is hard to truly value here as it relies on the overall list style and combos in play.

3. Screen-and-shoot playstyle

Everyone^ loves charging across the table into high powered gunlines hiding behind multiple layers of chaff screens, while getting blasted by unleash hell.

Impassable adds to the efficiency value of screens in this strategy. Fewer screens can control more board space and protect the charge lanes easier. In turn, this provides for the screens to push up the board more (because, say, the flank is protected by a impassable terrain) and/or allows the gunline player to invest fewer points in screens and more points in extra dakka instead!

The key winners of this are of course any Order screen-and-bang shooting lists. Because that playstyle is course a plucky meta underdog that needs all the extra help they can get (/sarc).

Meta Losers:

  1. 160mm base melee models (Archaon, Mawkrusha, Allarielle)

Pie Plate size models become extremely difficult to use with impassable terrain comp rules. While "defensible" terrain also does block their movement, defensible can at least be destroyed.

It is also worth noting that GW official rules only provide for 6" gap between terrain pieces. 160mm = approx 6.4". If there is only a 6" gap between 2 pieces of impassable terrain, then a 160mm base model literally has no where to land and is totally bottled up.

2. Large base non-flying models (eg Kragnos, Stonehorns)

While flying models can at least try to jump over any impassable terrain, non-flyers don't even have this option. Any large base, non-flying model already gets clogged up wasting movement to crawl over terrain, but at least it has the option to go up and over. Adding impassable removes the option and nerfs these models.

3. Melee only armies

This is merely an inverse of the above insight that screen-and-shoot benefits. The benefit to screen-and-shot comes at the cost to melee armies becoming less competitive viable or efficient.

Overall - the general theme of key winners/ losers is that impassable terrain simply aids the shooting meta further at expense of the melee meta. The game is (arguably) already biased in favour of shooting at expense of melee, so adding impassable just further fuels the current baises within the game. So, Mr TO who wants to run more shooting/ less melee what you are trying to do?

I Really Want to Add Impassable.... what should I do

With all that in mind, impassable does add a new tactical element to the game. However, if you are using impassable, then several things to bear in mind:

1. Announce it before list submission!!!!#

Adding impassable is a huge "meta changing" rule comp. Provide your players significant advance notice that you are going to have impassable terrain. If you don't pre-announce it, expect to receive a barrage of outrage from your Archaon and Destruction players....

2. 6.4" gaps.....

If you want to add Impassable Comp, also consider adjusting the GW terrain set up rules to include 6.4" minimum gaps between terrain also. While this doesn't fully solve the pie-plate issue, it is a minimum reasonable compromise to make the pie-plate models actually semi usable in an impassable terrain event.

3. Allow it to be smashed it down?

Consider allowing your impassable terrain to be smashed down as a monster action (potentially removed from play if so). This doesn't solve the problems created by impassable, but it does reduce the positive and negative impacts of it. Yes, this is more comp-on-comp, but, well that is the consequence of adding in such a large meta changing rule in the first place.

4. Points tweaks or unit bans?

Now this is where 'solutions' start getting a lot more controversial and difficult. You could considering banning Sevireth & Foxes from the event, or try to tweak points of the winners/ losers.... but this is getting complex very quickly and likely impossible to balance in a way that doesn't cause someone to be really upset!

Ultimately though, please bear in mind that the official rules do not include Impassable and therefore the impacted units points costs don't reflect the gains or losses to efficiency that come with impassable......

Final Thoughts

The implications of impassable are vast and meta transformational - frankly it is a totally different game to the official game.

Personally, I would welcome the eventual inclusion of Impassable to the official terrain rules...... so long as it is combined with relevant downstream rules and meta changes (eg 6.4" gaps, smash down style rules, and significant points changes or other rules changes for the positive/ negative impacted units, armies and playstyles).

Impassable is widely used in local friendly club meta's as well as TTS events, but in general real life major events have not included this due to the vast meta implications. The World Championships has now opened a door into using impassable (similar to opening the door into use of chess clocks!) and so there is potential for TO's to maybe begin more experimentation with use of impassable terrain rules at events. If so, well, the main aim of this article is to provoke debate and awareness on the issue and I hope that TO's that do include decide to impassable do so in a manner (i.e including relevant downstream adjustments to make it workable) that doesn't break the game.


*Defensible terrain (17.1.2) is impassable until it is destroyed (eg via a monstrous rampage). Various faction specific terrain pieces are also impassable (or defensible) until they are destroyed.

^By "Everyone" I of course mean "no one". Especially Peter Atkinson.

# As controversy, the World Champs use of Impassable terrain was only announced after list submission. This was a huge meta transformational rule change, and created massive outrage amongst the Archaon players and mass celebration from the Sevireth players. Unfortunately there were far more teams running Sevireth than Archaon, so there was no momentum to challenge this late call.

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