Attack from the sea

When revealed, kneel all locations.

Hellholt engineer

Response: After an opponent kneels a location, choose and stand a location you control.

I can stand X location of my own where X is the number of opponent's locations, right?

Miklos said:

No sir. The plot is what is actually doing the kneeling here. The opponent may be physically kneeling all of his/her locations, but the plot is what is forcing your opponent to do this.

Hellholt Engineer is used when the opponent kneels their locations to, for example, pay for a cost of an ability or effect. I am not sure if there are other voluntary ways for the "player" to be considered doing the kneeling, but for the most part if an effect makes your opponent do the kneeling, the Hellholt Engineer cannot be triggered.

Are you sure about this? I ask because it does not give any indication about why the location was knelt just that the opponent knelt it. Does this also apply to Joffrey Baratheon (KLE) "Response: After an opponent's character is knelt, pay 1 gold to stand Joffrey Baratheon. Then, he claims 1 power." as well then, because everyone I know plays it that when your opponent kneels it for a cost of an effect, because of a plot, or any of my own effects.

Penfold said:

Hellholt Engineer: "After an opponent kneels a location ..."

Joffrey: "After an opponent's character is knelt ..."

The verb form there make all the difference. "Is knelt" looks at the results of the action only. If the character ends up in the kneeling position, you're good to go. "Kneels," on the other hand, looks at the action itself. And because this effect specifies that an opponent must kneel the location, and there is a direct interaction interpretation in this game, only kneeling the location for cost is going to meet the play restrictions.

What do I mean by a direct interaction interpretation? Well, take the location Lost Oasis, which gives a Martell character Stealth, and the ability to kneel the character it bypasses with Stealth.

1. Does the Stealth kneel the character? No, of course not. The Stealth just creates the Response opportunity. It is necessary, but it doesn't do the actual kneeling. It's an indirect interaction.
2. Does the location or its effect kneel the bypassed character? No, of course not. The location gives the character the ability, which is then triggered separately. The location is necessary to create the ability, but once that ability is created, it's irrelevant. It doesn't do the actual kneeling. It's an indirect interaction.
3. Here's the tricky one. Does the Martel player kneel the bypassed character? The answer is actually "no." The player decides to initiate the effect that, when resolved, kneels the character, but the player's choice is not what resolves and makes the character kneel. It's just like the "when revealed" text on a plot being passive, even though the player chose which plot to reveal. Look at it this way: if you cancel the effect, you do not undo the player's choice to trigger the effect, even though the character doesn't kneel. So the player's action of initiating the effect is actually an indirect interaction - as far as what is making the character go from standing to kneeling.
4. So does the character or its effect kneel the bypassed character? Yes, of course. As the character ability resolves, it forces the bypassed character to go from standing to kneeling. So that's the direct interaction.

So the only time when an opponent directly causes a location to kneel is when he uses that location to pay a cost. Otherwise, he is acting indirectly though a card or rules effect. And Hellholt Engineer does not say "after an opponent's effect kneels a location...".

Nevertheless, Lost spearman, Hellholt engineer and Dornish fiefdom works like a charm then.

After you play Lost Spearman from your hand, any opponent may kneel 1 influence to return him to your hand.

I use Fiefdom to lower the cost of Lost spearman (to 0). Opponent kneel 1 influence to send it back to my hand (assuming that they use only location influences). Fiefdom stands due to Hellholt engineer's ability. Then I kneel Fiefdom again ... etc.

Miklos said:

A fantastic means to get them out without worries! :-)