On the basis of this question about teleportation circles, it follows that a simple casting of Dispel Magic could spoil a year's work for a player, as well as any setting established by the DM where teleportation circles exist in the cities. Dispel Magic has a range of 120 feet and requires only verbal and somatic components, something that could easily be hidden from those who could keep the circle. It would not be too difficult in itself to dispel this circle and get it, actually cutting this place for a whole year.
The text of the flavor suggests that circles are common in important areas.
Many temples, guilds and other important places have permanent teleportation circles inscribed somewhere within their boundaries.
Although it's simply a suggestion for the world you are building, this poses a potential problem, perhaps also a hole in the plot of your story. What is preventing a rival force from dissipating the circle and teleporting? Or an act of stupidity / intoxication at random? The circle is incredibly vulnerable according to the usual magic. It would seem that each circle should be secret and the idea of a publicly accessible urban circle would be totally unrealistic.
So, how do players and DMs make the circle difficult to eliminate with RAW solutions? Watch spells, special guards, magic items, infrastructure, all that is available in rule books. But if these are too wide, I can reduce them.