Editing Warhammer 40,000/7th Edition Tactics/Cult Mechanicus (section)

==Building your Army==
Let's be honest here: Kastelans are Derpy Baymaxes, and /tg/ is still [[Skub|split]] on whether we love or hate Kataphrons. For the Kastelans, the obvious solution is to buy the much nicer-looking ((But expensive)) Castellax from Forge World, though this raises the issue of how you're going to get your hands on a Datasmith. Short answer: Ebay. Long answer: make your own, or you can probably sub in a Techmarine with a Power Fist (FW's Astral Claws Master of the Forge and Iron Father both offer excellent opportunities for conversion, and are nice models in their own right). Paint him all red and say he's just a Magos in Power Armour if your opponent raises his eyebrows at you.

Kataphrons, well, if you don't like them, you're pretty much shit out of luck. You'll need to go third-party. [http://hitechminiatures.com/8-bio-tech-covenant| Hi-Tech Miniatures'] Bio Tech Covenant offers some excellent Tech-Priest themed characters, as well as a set of dudes with tank-tread legs. The only issue is that their weapons don't look all that 40K, but a bit of conversion work should fix that for you. Alternatively some ogre models with necron and general tech-bitz make quite good stand-ins if you don't mind a spot of converting.

Model choices aside, the first thing you need to do is decide whether you want to mainly use the Battle Congregation or the Combined Arms Detachment (CAD) to represent your army. Relying entirely on formations, while possible, is not advisable for this army. Your formations are good, but not amazing.
The Battle Congregation adds flexibility to your army. A WT reroll on the Cult Mechanicus chart is pretty nice, since all of them are at least decent and the recycling of a Canticle might win the game. In fact, it not only provides you with a wider range of options, it also provides you with the chance to use a particularly live-saving Canticle twice. It also has a list of choices specifically tailored to your army, with no Fast Attack and four Elites slots. In exchange, you only get 1 HQ and up to two Heavy Support choices. The last one isn't so bad since there are two great formations that use Castelans and if you have the points to spare for three units of them, you'll probably have the points for one of those formations. The 1 HQ, however, stings really hard, considering how obscenely good the Tech Priest Dominus is. 
The Combined Arms Detachment has three Fast Attack slots you cannot use and "only" three Elite slots, but its benefits speak for themselves. First, you may now take up to two Tech Priests, which means twice as many guys to hold your Relics and twice as many unkillable all-rounders. Much more importantly, the WT reroll isn't bound to the Cult table here and, even more interestingly, your Kataphrons gain Objective Secured in the CAD. They are shooty, so they can just camp on objectives and they are impossible to remove for anything else with Objective Secured.

The second thing you need to decide is whether you run Cult Mechanicus solo and if not, who to ally with. 
Solo Cult Mechanicus is, frankly, nowhere near as viable as solo Skitarii. You have no Flyer, no serious Anti-Air, nothing even resembling a Tarpit, no Psychic Powers (and a bit of Divination would ''so'' help this army out), nothing that moves faster than Infantry and your best anti-Horde option lies with the extremely close-ranged Corpuscarii (Incendine Combustors are only one per Castelan, so far too few to be viable). 
Luckily, Cult Mechanicus is pretty impressive when combined with something else. Give your Space Marines a big honking extra load of firepower, provide a Fulgurite meatshield to a Grey Knight Librarian for a gigantic blob of teleporting 3++ S7 maniacs, add on a bunch of Guardsmen and Valkyries to solve most of your problems neatly, it's your rodeo.
Best of all, combine them with Skitarii for the best Anti-Air of the Imperium, a bunch of Vanguard and a few Sydonian Dragoons with Radium Jezzails because fuck those characters, heavy weapons and MCs.
Also, you know, Divination from a Blood Angels or Dark Angels Librarian couldn't hurt. Kataphrons work extremely well with Prescience, Perfect Timing, Foreboding, Forewarning... you get the point. Keep a bunch of Skitarii Vanguard on hand in case the Libby gets Misfortune.