We found out about the ruins only AFTER colonisation (Dorthonion seems to have been founded in Y2). (Even if we knew about ruins, it wouldn't have been too much trouble to send in a relatively small scouting force to check before the place was levelled to the ground.) Doriath is the only city noted to be built on a mountain and the fact that it was built on a mountain doesn't mean we didn't clear it with ortillery first.
There are two alternatives to thinking we cleared the place with ortillery. The first is that we had a large number of troops, materials, and civilians march to the colony targets through many kilometres of psychic death world forest. (No.) The other alternative is that we airdropped them in from above, in which case they would've had to fight and clear the ground right after touching down, taking loads of casualties as they tried setting up fortifications as quickly as possible, then clear-cutting areas around those fortifications to create a no mans land that they can easily see and shoot into. This second alternative is just the ortillery option except slower, costlier, riskier, and with a hell of a lot more casualties.
And the alternative is that we nuked the areas from orbit, an action which is not only inaccurate, but is also very likely to do away with the colonisation prospect in the first place. We choose areas like Holin and Helms deep for good reasons, nuking em from orbit seems like an excellent way for them too loose said characteristics, like say blasting the fertile valley of Helms deep.
Also Doriath is the only city in Lindon built in a mountain, but its far from the only one on Avernus. Cassonadro for example was primarily built into the tunnels left by a massive Gnaw worm, the kind of things that would collapse if nuked from orbit (the kind of stuff that did collapse after we later nuked it from orbit due to chaos cultists.)
What seems more likely is that we did what we do now, first surveyors to find good spots and find any dangers preemptively (its how we knew
not to make a city on the giant turtle) then send in our professional soldiers to set up defences (the first thing that was always set up) and then finally here comes the civilians to create the actual city. Its what we did in Duat, first establishing forts which we held and then using said forts as the foundations for cities and I find it unlikely that we'd not use similar tactics for the other cities, especially when each region had soldiers that were often experts in the region being colonised (Catachans for Lindon) and thus could help build the fortifications much faster.
To clear it up though
@Durin
1. Would we bombard perfectly nice city sites from orbit?
Eh, more a minimum of 70% casualties. We managed merely 30% casualties, which is likely better than most of the safer Imperium Era colonization efforts not supported by super elites (Astartes, Myrmidons, Knights) of a mien amenable towards civilian survival. And yeah, we discovered the ruins after we started colonization. Technically when the Emps died, pretty much every organization with a High Lords representative (sans Astartes and possibly the Custodes) were ripping through the Pacificus command structure for the sheer utter stupidity about the planetary ruins alone, and were likely in a frothing rage when they learned of the Lunar ruins and the MASSIVE ship graveyard. All of which the surveyors (who did deep searches of the system on Inquisition orders) somehow MISSED. This is like no one noticing Roboute out and about visiting the Cafes of Ultramar for a nice cuppa, let alone not being in his stasis pod.
Kinda, I presume the initial surveyors that were got by Klovis merely looked for a world that's suitably dangerous for his special project, although missing something like that is kinda dumb, but equally they weren't looking for city sites or technology, so we probably found them when we sent surveyors down to find said sites (like the ones who discovered the turtle).
Still, them missing it...surprisingly I can say that it's somewhat plausible.
1. We know Avernus is supposed to be harder to find in the warp than a normal system. Not like near invisible, but just harder to find. Given that nothing interesting is supposed to be that way I'd be unsurprising if many navigators just pass it over.
2. The Asgard subsector as a whole is both waaaay off the beaten path (its on the far edge of Segmentum Pacifus) in the segmentum that most consistently goes into rebellion against the Imperium. So its possible for news of this discovery to have missed the high lords potentially hundreds of times.
