The only military threat Grantville could fear was a cavalry raid, (as the slow-movig tercios had no chance against modern guns). So why don't they build at least a wooden palisade around the town? Or at least around the power plant and other important buildings? And why don't they make at least rudimentary flamethrowers? In a world dominated by close combat, they would be much more useful than even the best modern hunting rifles and shotguns.
"Rudimentary flamethrowers" is easy? Are you kidding? Even actual military grade flamethrowers could be dangerous to users and could still malfunction. A rudimentary flamethrower assembled by people with no experience building them would be even more dangerous and prone to malfunction. And the 17th century is hardly dominated by close combat, unless you consider marching in ranks and shooting muskets "close combat." There's only a limited amount of modern weaponry, so later in the series most warfare still uses ranks of troops firing single-shot rifles. A short-ranged flamethrower is useless in that style of warfare.
Cavalrymen aren't tied to their horses. It would take around the same time it takes for a dozen men to dismount and hack through or pull down the palisades for it to be rendered a moot point, and the amount of labor and lumber spent building one isn't really worth it.
Obstacles were used in real life to stop cavalry forces, which once off their horses would be far easier to handle.
If the "Ring of Fire" the town is in is at a depressed altitude from the land around it, why doesn't it just turn into a lake?
It's not at a depressed altitude. There are differences in terrain that make some portions jut up and above, some portions are below, and some are more or less on level.
But the cliffs are frequently described as forming a ring entirely around the town six hundred feet high — there should still be a substantial area below the lowest point where it can drain.
The average elevation is about the same as before, but the topography doesn't match up, so there are cliffs where a hill on one side of the ring faces a valley on the other. Several of those valleys are drowned. Fortunately for the town, the lowest spot on the ring, where the main creek drains out, faces even lower ground outside. You can see maps of the result here.
Is it just coincidence that the 'new Americans' of Grantville come from two ethnic groups that made massive contributions to the Original America? Germans and Scots that is. Whatever the horrors Germans got up to in their own country in this dimension they made great Americans.
Remember the beginning of the novel, where the Abrabanels get rescued by the Americans? This obviously doesn't happen in the original ("our") timeline. There, the Imperial soldiers caught them... (Likewise, the biography of most other downtimers who did find a save haven in Grantville would probably be a far more gruesome read in the original timeline.)
They were all dead before the Ro F, per the prologue to 1632. Seven bodies were found in the Ring, which seems about right; it weas sparsely populated farmland in a warzone.
Time Spike strongly implies that the US government has them in custody and is trying to hush the whole thing up so they can blame the incident on terrorists. It also strongly implies that the coverup is going to be blown out of the water in short order after the Illinois incident.
In 1633, when he learns that his navy is going to have to engineer the defense of Wismar against the incoming Danish fleet, Admiral Simpson assigns Lt. Cantrell to determine what resources are available to send. Somehow Lt. Cantrell is able to do this all the way from Magdeburg, presumably by radio ... presumably because he already visited every town in the house in "Curio and Relic" as Paul Santee's Assistant Weapon Scrounger, and therefore already has a sense of the social landscape of uptimers in Grantville that's far more detailed than almost anyone's.