Why have a teleport circle near your home base?
Maybe because it makes teleporting less expensive by a large factor? Or maybe because it lets you return there quickly?
Sure, it can be a weak point - if you let it. But nobody says you have to put the circle in a convenient invasion location. Try outside but nearby your base, surrounded by traps that only you have the key for. Or if you want an "exit only" circle, use the new Teleport Catcher ritual and shunt anyone entering the circle to a pit of lava.
And while it may be possible that the circles are common enough for most cities and guilds to have them, and yet are all relics of ancient times that nobody can make anymore ... its rather unlikely, and more importantly, it feels like disempowering the players. "Nyah nyah, NPCs can make teleportation circles but you can't, you better just go where I tell you".
There are few enough ways left for PCs to shape the world in 4E - at least let them pick where they want to travel.