I learned an interesting thing about the editor and the game that differs from what I previously thought: if you want a specific spell to appear only in certain towns, you need to:
1) enable it globally in the map settings;
and
2) set it as required in the properties of those towns (the "Spells that MUST appear in the Mage Guild" list);
and
3) disable it for all other towns where it should not appear.
In my opinion, this is not very convenient (and not entirely obvious, as I had a different understanding of the meaning of the words "MUST appear"). That is, if I have 48 towns on my map and only one should have, for example, Summon Boat, then I have to manually disable it in the other 47 towns.
At the same time, if you globally disable certain artifacts on the map but place them in some locations, they will appear as expected.
For example, in my test "blank" map, even though I set City Portal in one town, something else kept appearing there until I started generating starting saves and figured out what was wrong.