Thank you Broomstick, but I never got so far as to get a license number.
Waiting for the pro version, I downloaded the free 8, and got a slight surprise: the free 6 that I had imported dxf's, the 8 doesn't. Luckily I didn't delete the 6, so it's ok.
I've been thinking a bit about the connection between the atrium and the system on the Oppio, termae of Titus etc. The Velia hill (I drew the top as an hexagon) was 42 m. high, the Oppio about the same, 40 or so, but in between there was a valley, the Carinae, that were contiuous with the plain that is now the Coliseum square. In that area, huge neronian walls were found, that serve no apparent purpose but that of creating an artificial hill that filled the Carinae and connected the atrium with the termae of Titus. So I suppose that the atrium was just that, an atrium, with connections to the other wings of the Domus Aurea. I placed a simple porticus, that climbs slowly from the 30 or so of the Atrium to the 38 of the termae, I have no cue of what it could have been like, could even have been a much more important structure. I wouldn't be surprised if there was another such connection between the atrium and the structure on the Vigna Barberini with the rotating tower, it would make sense.
Extended the acqueduct to the Palatine hill. It has a slant of 3 per 1000 to ensure water flow, so it lowers about 30 cm every 100 meters ...

These are the houses of the neronian period found in the area excavated by my wife.

Ok, Csaba, I'll swallow my annoyance and tell you what happened, maybe it'll help me to be less mad about it. You see, I live in Italy, a wonderful country, but with a bunch of thiefs in the government and services that don't work. Last year I bought a release of Poser and the manual. They sent me both by postage, the book arrived, but the box with the software was stolen. Normal for our postage service. So I repayed for what I had already bought, this time the downloadable version, and ok.
Now I wanted to buy an educational version of Sketchup Pro, so I went to the Sketchup site, and the only way to buy it, even if it is a downoaldabe software, and even if I prefer to work with the english version, is through the italian representative. Just one in all of Italy. The italian representative doesn't accept credit cards or Paypal, a hell of a way to do e-commerce. So I had to go to the post office, I payed, and was stupid enough to misplace the receipt, so I can't find it. After a week or so I mailed the agent to inquire what had happened with my software, and he mailed back he had never received my payment. No way to trace the payment through the post office if I don't find the receipt.
So on monday I'll go back to the post office, pay again, and keep the receipt in a safe after sending a scan to the vendor. So sometime in the near future, I hope, I'll have the d.. Sketchup pro version 8....

Third thing: I am working with different orientations, sometimes only slightly different. Setting the axes each time is time consuming, and it is easy to make mistakes. Is there a way to save axes? like Axes Domus Aurea, Axes Via Sacra etc.? And them reset them.

Thanks, Broomstick (suppose you need a biiiiig broom in Naples these days, wish you luck). I welcome any help that can be had.
This is my try at the cenatio rotunda, waiting for comments from Elysium. The tower was found up to platform level, it might have been razed by the flavians, or it could have been a wooden structure, the logical solution for a rotating hall.
Couple of requests. Firs thing. As you may have seen, I gave up (at least for the time being) to the idea of starting with the DTM's. I found it easier to set the buildings, place the terrain level we know of around them, and reconstruct the lay of the land from there. Of course this way the terrain keeps changing. The question is, it should be possible to make a layout of the road system on a flat plane, and then project it as needed on the terrain, so as to place the right texture and limits to the roads. I think the sandbox has a command that should do that.
Second, stupid thing: the measure instrument should "remember" the last measure used when placing a guideline, and repeat it on the next. I get mixed results, some time it remembers, sometime not. I must be missing something.

Just to show that something is still standing, and that not everything is an invention. The acqueduct does a sudden zig zag to overpass the road. It's known as the Arch of Dolabella. Of the acqueduct that carried water to the Palatine only the two lower tiers are still standing.

Deleted, you are right, not too smart.
On the whole I agree with Pichuneke, I have tried (and will keep trying) to use Blender, but the learning curve is quite steep. Poser can be had for about 200 bucks, and several weeks of my time are worth more than that. 3D Max is in another price level (I use Design Cad, quite cheap).
I have started fighting with the acqueducts. I think they sort of blend with the back side of the Claudianum (my wife doesn't agree, we'll see).

I don't think you can beat Poser for that. But of course you get zillions of polygons, I'll be using it for some renderings, but not for the overall model. I'll need some texture that looks more weathered though ...
As for the images of the Domus Aurea, maybe I'll do some interiors after all, if you can send me some walls to ... , I'll be grateful ...

Thank you, Pichuneke, but for the time being I am not planning to do any interiors, got my hands full with this scale. And I have done some things before with the two guys who did that fine work in the first video (Borghini and Carlani), so if I ever get to that I'll call them.
This is what I've done so far. Uffff! Now the west face and the acqueduct, and the south face too ...

This is another video, much less reliable as reconstruction, but it's nice. Altair has some other ones worth while seeing.
http://www.youtube.com/watch?v=U4zVNmQWZjg
This is what the two accesses to the lake positioned symmetrically look like. To modify something is hell, I think it's ususlly easier to start again from scratch ...

For the Domus Aurea, this video by Viscogliosi & Co is one of the best things available:
http://espresso.repubblica.it/multimedia/5289136
It has some imprecisions, for instance the temple wasn't on the Claudianum until later, but on the whole it's impressive.

Thank you. Martin Conde is a great guy, and my wife has relied on him for a long time to illustrate her excavations of the Meta Sudans.
Here is another image to explain the relevance of what I found.

I made an interesting discovery, examining with more attention the map of Lanciani. He registers a XVII century document that shows a sudden change in direction in the retaining walls of the Claudianum, aiming in a perfect perpendicular to the lake. If we take that into account, the access on the lake becomes almost perfectly symmetrical with the access to the termae of Titus. It can't be just a coincidence. I'll work on that.
Thanks Pichuneke (my basic language is spanish too, but let's stick to english), The Coliseum you linked to in your mail is really impressive, like all the works of our friend.
As for the pool, I don't think it fits. What I had in mind were some large stone pipes, spewing water with force into the lake. The water comes from an acqueduct straight from Porta Maggiore, and by association I thougt of the tomb of the breadmaker ...

And here it is what it looks like when placed where it belongs. Lots of things to revise. The residential part is a bit too spiderlike, the channel too straight, the staircase too imposing, the connection with the lake unresolved, and so on ... Suggestions welcome ...

I started with the Claudianum, and it won't be easy. Nero's mother started the Claudianum as a worship area for her deceased husband Claudius (probably killed by her) but the temple was just growing at the time of the fire. I am now trying to model Nero's situation. It was an important part of the Domus Aurea, sort of a leisure residence, with gardens and water games. He razed what existed of Claudiu's temple; after his death the flavians razed his residence and rebuilt the temple.
The acqueduct Celimontano arrived at its back, and gave it plenty of water. After being used for fountains etc. the water went down to the lake, probably through a monumental fountain. The Claudianum stands at 49 m., the lake at about 21, so there are 28 m. of dislevel. A piece of a marble ship bow with a boar's head was found there, so I imagined a whole stone ship, much like that in the Tiber island.

Changed some details on the lakefront ...

Muchas gracias, Pichuneke. Pero te confieso que la cosa se està poniendo difìcil. Ya veremos.
This is what I ended up with for the access to the Termae of Titus. And it took me a lot of work too ...

This is what I had in mind for the connection. The main function is as struts, to keep the terrain with a 12 m. dislevel from falling over. The Via Labicana would go through the double arch overpass. Two lateral ramps go down to street level, and one goes on the overpass to the lake. Of course this is roofless, to show the pathways. Walls and roofs will monumentalize it.

Well, it's growing. Now I have to solve the connection between the termae and the lacus, and with the Via Labicana. I'll try a solution with an overpass, it's easy since the huge staircase allows for it, 12 meters of difference between the courtyard and street level. But maybe that's too modern ...

Started with Titus's baths. Extruded Palladio's reconstruction, but some things are fishy. I think the whole thing is aligned with the rest of the Oppio wing, they were initially intended (probably) as Nero's private baths. Don't think they were that symmetrical, the slant on the right side doesn't make sense. And the front abutting on the lake shore (or later in front of the Amphiteater)must be rethought ...