Strictly speaking, neither Hommlet nor the Keep are "towns," they just aren't big enough.
For an intro they should include a region; something smaller than the classic Duchy of Karameikos, perhaps, maybe just the "Borderlands Region" that can be dropped into any existing campaign setting. You could have the Keep on the Borderlands right there on the border of the savage wilderness near the Caves of Chaos. A day's march down the road the Town of Threshold, a bit further toward civilization, but still in the Borderlands... it's a place where things are wild and wooly, and a mix of civilized and savage; characters can re-equip and rest, and yet still have "urban" adventures. And then a bit further down the Road to the Border you have the Village of Hommlet, sitting there in the section that is civilized... mostly... as raids have been increasing on the border lately, they are now building a keep (maintained by a fabulous pair of former adventurers), and strange things are now happening at that old moathouse that was ruined when Hommlet was on the verge of the wilds...
There. An entire mini campaign setting with a town, village, keep, caverns, and even a ruins, if you use and slightly expand the old moathouse... That's what, 16 pages, maybe? Perfect for the DMG...