In Hadrian's day the marshy flatland wasn't even healthy (malaria and other fevers), although he certainly used his wealth to clean up the area, raising terraces and installing well planned drainage channels. He also installed great water works, including pools, baths, and fountains, using cooling waters he piped down from Tivoli. Following long established custom, he added fragrant oils to his aquatic installations and, although unwittingly, thereby prevented them from becoming breeding areas for mosquitoes. (Unfortunately this practice has lapsed.)
For years, archeologists and architects claimed that the villa was built late in Hadrian's reign (117-138 AD) and was an attempt to memorialize foreign architectural sites he had come to love during his extensive travels as Emperor. Now it is clear that the villa was started probably within a year of his accession after Trajan's death. That only means, however that he had already imbibed a great deal of foreign architecture before assuming the purple and during his early imperial years. Despite the efforts he lavished on the Villa's design and construction, which lasted until 134 AD, Hadrian didn't spend much time there. He traveled extensively throughout his reign and died at another villa at Baia on the Bay of Naples.
Like the Pantheon, another of his construction wonders, Hadrian's villa was revolutionary in Roman architectural and artistic terms. And Hadrian was his own chief architect. Instead of the rectilinear plans that had preceded him, Hadrian used curves and, above all, domes instead of barrel vaults. Some of this was incorporated in later bath structures, but much was soon forgotten and not really used again until the Baroque period a millennium and a half later.
Hadrian also designed a vast network of chambers and tunnels, some of which were clearly too sumptuous for use only by servants, underlying and interconnecting the many buildings at the villa site. They were probably cooler and also almost certainly funneled cool drafts into surface and upper rooms (draft tunnels, "wind towers", underground fountains, and domes pierced by occuli, all designed to augment normal air convection, were already in use in the Middle East and may have been consciously copied here.) Recent site renovations have made most of the underground works unavailable to all but the most determined and devious tourists.
The archeological site is huge and it takes several visits really to explore it, although a very enjoyable cursory tour can be accomplished in a half day (perhaps using the other half to see the gardens and fountains of the Villa D'este (closed Mondays) in Tivoli). The underground portions aside, a great deal of the villa is now accessible using recently renovated pathways. Maps, guidebooks, and AudioGuides (English, Italian, other languages) were available at the ticket booth. There are a good visitor center, a small snack bar, and clean restrooms near the entrance, and the museum next to the Canopus was reopened in 2000 after a long closure for renovation. Some replica statuary has been erected around the site to replace pieces in the site museum and the many pieces hauled off in past times to other Italian and European museums. The back side of one of the baths was used as a theater and pop musical stage during the summer of 2000, and more performances are envisioned for 2001, although I haven't yet been able to find a schedule (ask travel agents in late spring -- take a tour bus to a show rather than driving.)
Getting to the site, 28 kilometers from Rome is fairly quick and easy. It's just off the Via Tiburtina, four kilometers before you reach Tivoli. You can take Via Tiburtina or the A-24 to the Tivoli area, but if you use the A-24 you miss both the sulphurous odors of the baths at Bagni di Tivoli and the spectacular views of the huge quarries from which most of the Travertine that built Rome was extracted.
Hadrian's Villa is well represented on the Internet:
A general site with map and pictures: http://spruce.evansville.edu/~ic2/.
Tourist info from Capitolium.Org: http://www.romapreview.com/pagineing/vedere/adriano.html.
A New Zealand school site with lots of info and two good reconstruction views: http://www.chch.school.nz/mbc/hadrianv.htm.
From greatbuildings.com: http://www.greatbuildings.com/buildings/Hadrians_Villa.html.
Construction history and guide: http://www.alfanet.it/welcomeitaly/roma/itinerari/tivoliing/tivoli2ing and linked pages.
Pictures from Art Survey: http://rubens.anu.edu.au/htdocs/laserdisk/artsurvey/bysite/display00497.html. (Use "next page" links at page bottoms for several more pages of pictures.)
Hadrian was the middle of the five "good emperors" (Nerva, , Hadrian, Antoninus Pius, and Marcus Aurelius.) His bio from Britannica.com is at: http://www.britannica.com/bcom/eb/article/7/0,5716,39537+1+38745,00.html. His bio from De Imperitoribus Romanis: http://www.roman-emperors.org/hadrian.htm