[Zigûr]

Originally Posted by Morsul the Dark

I don't believe Gandalf was there too long.

According to the Tales of Years, Gandalf was imprisoned on the 10th of July and did not escape until the 18th of September. As such, he was imprisoned atop Orthanc for over two months.

However the text does say the following:

'They took me and they set me alone on the pinnacle of Orthanc, in the place where Saruman was accustomed to watch the stars. There is no descent save by a narrow stair of many thousand steps, and the valley below seems far away. I looked on it and saw that, whereas it had once been green and fair, it was now filled with pits and forges. Wolves and orcs were housed in Isengard, for Saruman was mustering a great force on his own account, in rivalry of Sauron and not in his service yet.'

This suggests that the pits and forges were already in place when Gandalf arrived, but I suppose it could be argued that "I looked on it and saw" does not necessarily mean he looked on it immediately; he may not, in fact, have been able to do so:

'Over all his works a dark smoke hung and wrapped itself about the sides of Orthanc. I stood alone on an island in the clouds; and I had no chance of escape, and my days were bitter.'

Perhaps the work only started after Gandalf arrived, but when he was first taken to the pinnacle of Orthanc he could not actually see to the ground.

It's also possible that when Gandalf refers to "the valley below" he is exclusively referring to the vale of Nan Curunír beyond the Ring of Isengard itself. Upon riding up to Isengard he may not have been able to see the work going on; presumably at least some of the valley was behind or to either side of Isengard proper, and thus any works going on there might have been hidden from view on the ground.

Note that when the characters travel along the approach to Isengard in Book III, no forges or industrial works are mentioned:

most of the valley had become a wilderness of weeds and thorns. Brambles trailed upon the ground, or clambering over bush and bank, made shaggy caves where small beasts housed. No trees grew there; but among the rank grasses could still be seen the burned and axe-hewn stumps of ancient groves. It was a sad country, silent now but for the stony noise of quick waters. Smokes and steams drifted in sullen clouds and lurked in the hollows.

While these might have been destroyed by the Ents by the time of their arrival there's no particular suggestion of it. The Ring of Isengard itself, however, is described thus:

The plain, too, was bored and delved. Shafts were driven deep into the ground; their upper ends were covered by low mounds and domes of stone, so that in the moonlight the Ring of Isengard looked like a graveyard of unquiet dead. For the ground trembled. The shafts ran down by many slopes and spiral stairs to caverns far under; there Saruman had treasuries, store-houses, armouries, smithies, and great furnaces. Iron wheels revolved there endlessly, and hammers thudded. At night plumes of vapour steamed from the vents, lit from beneath with red light, or blue, or venomous green.

Perhaps the work began in the valley outside the walls, hidden from public view beside and behind the fortress itself, and the inner "plain" of the Ring of Isengard was only exploited later; perhaps then the work began while Gandalf was present.