On some levels, I know that this isn't completely an appropriate for this forum, but it does directly have to do with the plots of Only War's settings, so I will ask, and hope that either OW people know, or RT knowing people also visit here.

So, I am Duke Severus, hiding out in my massive fortress on Kulth. What defenses does my planet have against spaceborne threats? Nothing repelled the Ork Waagh! from making planetfall, and if it had been something more like Nids, I expect that there wouldn't be anyone left. Thus, why hasn't the Calixis Battlefleet flown in a few big ships, and opened fire upon his castle? Or hired a Rogue Trader loyal to the Imperial cause, if their schedule and personal logistics might be a bit more flexible? I might assume that the castle has some void shields, and there might be a few macrocannon emplacements, but still, it doesn't seem it should be too hard to manage. If I look at Lure of the Expanse, a Zayth land-ship has effective stats on p.65. Assuming Severus's castle isn't considerably better then a moving hive city in an active for generations warzone, and can move even less (as in can't), what makes it a hard target? I wouldn't destroy the planet, but would think knocking over one building shouldn't be too difficult.

On a slightly unrelated note, also why is Severus still alive? I have a plan in mind for if a group of my RT players, if ever I have such a group, become TOO heretical/secessionist, they will be infiltrated by a Callidus Assassin. Unknown to them, they will pick up one in port, looking like any rating, or what have you, and it will slowly infiltrate the party. To reflect the skill with which they play, I plan to let the character she is impersonating keep playing, unknowing that they are actually not there. Then, when the time is right, I purloin their sheet, spring the Callidus, and eliminate the Rogue Trader. The faked player then learns that they have been locked up in a crawlspace, or what have you. it's a ****** bag move, no question, but fitting for the Callidus, and I would have given them plenty of subtle warnings by that point; the Imperium doesn't hand you all that power, and then just send you off to give them the finger.

So, to me the plan seems not too far fetched. What is supposed to have stopped any OH Inquisitor in that sector from having already done this? Severus is now an open and declared secessionist and traitor, and the Assassinorium was rather made for this sort of thing. What might be the hold up? Please be a better excuse than "the Imperium needs to let him live so that they can keep up the ruse that covers the real deal, the Jericho Crusade."

OK, I'll toss out some theories...

I've seen background material related to massive land-based orbital defense batteries of the kind that can take out a cruiser. I think we can reasonably assume that Duke Severus's capitol, in addition to massive void shields, is likewise defended by such batteries. Sure, you can make planetfall somewhere on his world but not on the capitol. Likewise, any ship moving into position to bombard the capitol will be fired upon and driven off or destroyed.

As for just assassinating him, I can think of a couple of less solid reasons for that. Firstly, Temple Assassin's aren't actually a dime-a-dozen. Realistically, I think there might be one or two in the whole Calixis sector. It may well be that whoever assigns them to their tasks doesn't consider Duke Severus a target worthy of one. It may be that the only available assassins are already assigned to other, equally critical, missions and when one becomes available it will he sent after the Duke. It may be that one is in the process of infiltrating the Duke's inner circle "as we speak" but has yet to get close enough. Perhaps the Duke has a cadre of elite bodyguards (perhaps including psykers) who have protected him from assassination attempts.

A small force of elite Imperial Guard troopers and support specialists might be ideal for sending on missions to either cripple the Duke's orbital defense batteries or provide support for a Temple Assassin be deployed to take out the Duke. Of such things, adventures are made.

Severus' doom seems pretty assured at this point - it may take a few more years to finally crush him, but everybody's going the right way about it. As for the Inquisition and the Assassinorium; any that are hanging about the Front, rather than passing through to Scintila/Reach/Expanse/wherever, have bigger fish to fry - such as Warboss Gitslaver and the DE Archon.

Also, the Orks got past Kulth's defenses simply by providing more targets than the defenders had guns to shoot them down with. A full blown Waaaagh! lacks for subtlety, but little else.

I can see that. I just sort of thought that, with such threats being present, and the Imperium being unwilling to commit the forces necessary (the Jericho Meatgrinder is a demanding, if occasionally pointless-seeming enterprise), the elimination of the Duke, and subsequent reabsorption of the mostly loyal soldiers (many of Severus's men still mostly serve the Emperor's cause, even if their leader is misguided) in his force would be high-priority to keeping the Calixis Sector safe. While any of several terrible things could come through the JM warp gate, should the Crusade there totally fail (I'd blame Ebongrave), the forces of a full Ork Waagh! are already here and in full force. If the Spinward Front is at all close to the Calixis Sector, an area known for slightly weak worlds, slightly rife with Chaos, and no Space Marines to speak of, then that Ork force would seem a credible threat to them. Granted, I don't know all the stuff in SF lore, so maybe the Orks are content to hang there for now, and maybe the Duke will be dead soon, especially with whatever is coming in the next book, but yeah. I also might be giving the Callidus Assassins a smidge too much credit, but I like to think not

Some of this, of course, must stem from the "they need to keep material, so that the line can keep getting written" aspect, which I also get. Not being overly fond of the Duke and his forces, especially if they follow everyone else and just fall to Chaos, I over-see him as the weakest opposition, and that makes it harder for me to see how he hasn't been dealt with.

The other thing is, your assuming a rational, well considered and well planned response on the part of the Imperium. If real life history is any judge, governments rarely, if ever, do the smart thing and when they do it's usually only as a last resort.

Given the sheer bureaucratic absurdity of the Imperium, it wouldn't surprise me at all to learn that the Departmento Munitorium was still shipping regular supplies, weapons and ammunition to certain Dominate regiments because someone submitted that periwinkle copy of the "these guys are traitors" form instead of the goldenrod copy.

I could be totally off-base here, but given the fantasy roots of 40k, I've taken to thinking of space engagements and defenses as analogous to Age of Sail-era naval engagements and defenses. So, void shields take the place of walls, macro batteries are cannon emplacements, and space gun platforms take the place of harbor fortresses. The space stations would be positioned so as to prevent enemy ships from drawing a direct line of sight on the city/palace without exposing themselves to withering fire from other defensive platforms. If they're armored enough, the IG would have no choice but to invade the fortresses to open the path to the planet; even then, you'd have to contend with the planetary void shields and batteries.

As for why Severus isn't dead yet: the Assassins probably just haven't had a chance to get into position yet. The war is still pretty new.

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I could be totally off-base here, but given the fantasy roots of 40k, I've taken to thinking of space engagements and defenses as analogous to Age of Sail-era naval engagements and defenses. So, void shields take the place of walls, macro batteries are cannon emplacements, and space gun platforms take the place of harbor fortresses. The space stations would be positioned so as to prevent enemy ships from drawing a direct line of sight on the city/palace without exposing themselves to withering fire from other defensive platforms. If they're armored enough, the IG would have no choice but to invade the fortresses to open the path to the planet; even then, you'd have to contend with the planetary void shields and batteries.

What he said. Taking example from the Citadel of Vraks - one of the most detailled examples of a planetary invasion and siege we have in 40k canon - you can drop troops in all you like 'over the horizon' but if you're ever within line of sight of the citadel itself, you are toast. Remember that whilst zaythi landships are powerful, they're still mobile units, and designed without some of the technologies of the Imperium - whilst they have their automated batteries, they don't, for example, posess melta technology. The big driver is always described to be power . A static emplacement can have reactors powering a void shield massively more capable than anything mobile, and 'proper' defence lasers ( which have a similar effect on orbiting cruisers to lascannons on APCs) rather than just macrobatteries.

So, let's say I'm stupid; it's not always arguable. If I were a Rogue Trader, flying my light cruiser over Kulth, hoping to score points with the Navy, and the Imperium, as a whole, what sort of reception might the Duke's fortress offer? Any idea what sort of guns he might have? How many? How powerful should his void shields be? On the one hand, it might just look like I want to know what I have to be able to beat, but then, it also matters what all they have to face. Yes, the fortress could be a monstrous structure, sitting atop several plasma generators, each safely buried deep into the crust of a planet, and it could have numerous macrocannons, plasma cannons, or whatever the Duty's Fist in the older Apocalypse book (p.87) counts as; some kind of laser. Battleships seem to like 4 void shields, so a structure like this could have maybe 5-7? I really don't know how one might plan some of this. How many hits my ship might take, and how many before it starts to notice. In RT, there are several instances where the crew's only real incentive not to shoot the planet below is because that might destroy the opportunity that they have to negotiate with the douchebag for. Over Vaporius, one might think you could nuke a glass city, and force water out of another, but no. Only over Zayth do they actually try. You COULD fire down upon a landship, but she has claws, too. They tell me what the reception could be like. In OW, of course, that isn't the typical reception for the party to expect, being ground-pounders, but anyway...

The world of Arseendofnowhere, Koronus Expanse, is not a particularly good model for a proper Imperial fortress. Rogue Traders have the orbital gunfire card in their sleeves primarily because they never encounter a genuinely protected installation.

The term for the Apocalypse orbital gun installation (like the Duty's Fist) is "Defence Laser". It's not a question of the size of the weapon so much as the power supplies it has to draw upon. A land-city has starship-level macroweapons, but a Defence Laser is not ship-to-ship weaponry; the closest thing mounted on mobile platforms in power (abeit explosive rather than beam) is a nova cannon.

How many shots can the ship take before it starts to notice? One .

It's like a lance scaled up to 11. I think the Battlefleet gothic rules considered a single defence laser silo equivalent to a consolidated battery of about two to three capital ship lance mounts in firepower, and the Vraks citadel was sitting at the centre of a network of forty of them.

The Defence Laser silos on Vraks meant that a Dark Angels Battle Barge didn't dare manouvre into orbit. Similar weapons in black library novels regularly one-shot regimental troop dropships and light warships.

A not-quite-military grade cruiser commanded by a Rogue Trader would be turned to swiss cheese by the first volley.

Equally, fortress void shields in novels such as Storm Of Iron were able to hold up to the siege firepower of an entire Iron Warrors Grand Company, with no noticable ill effects. Equivalent installations in the Horus Heresy series have stood off city-wrecking bombardment that turned anything outside the shield into a nuclear wasteland. Certainly they would hold long enough under orbital fire that the ship is fighting a losing battle against the fortress' guns.

Indeed, the fluff makes clear that proper fixed planetary defences are capable of withstanding orbital bombardments. However, this requires a lot of power hence power stations which tap into planetary thermal energy sources.

Which leads us to the two typical ways in which the Imperium goes to war.

Firstly, surgical strikes by space marines (usually landing over the horizon and then moving quickly to their targets) taking out command nodes, power stations and anti-space batteries and missile silos, e.g. the typical space marine missions.

Secondly, grinding atritional warfare by the Imperial Guard (occasionally supported by space marines to take out particularly nasty defences) to fight their way close enough to either batter the void shields covering the target down by repeated artillery strikes or launch massed assaults.

Regarding the use of assasins, think back to earth's wars. How often are they decided by assasination? For starters, you risk relatiation in kind, i.e. Napoleon executed a Bourbon prince as a warning after which all assassination attempts against him stopped. Secondly, 'better the devil you know'. The allies in WWII actually preferred Hitler instead of a possible successor who might be sane and thus nixed a few possible attempts. Severus is a smooth political operator but not nearly as good as he thinks (Hax allegedly knew what he was doing all along) and not a very good general. Assassinating him could lead to a better successor and might well turn him into a martyr.

Or, The Assassination attempt could have failed! (It does happen!)

In BFG, A ground based defence laser is roughly equivalent to a lance battery.But who knows how many the capital city has a around it. Additionally, There will be Torpedo silos; Orbital fighter launch bays, Orbital defence stations, System ships etc. It would probably be safe to assume that a 40k sized Hive city will have roughly similar defensive capability to a Ramilies Star fortress (Which are significant!). Simply put, You are committing to a MAJOR Naval engagement in order to even get close enough to the fortress to Bombard it!

BTW: there is a Space marine chapter in the Calixis sector; The Storm Wardens. They probably are even engaged along the Spinward front but that doesn't mean the players will see them. If they do, it means the s**t has gotten deep indeed!