Come on! Stretch that imagination. How would you do it?
Let's run down the list:
Major Temple: This one is easy; put the
teleportation circle in the entryway to the chapel, off to the side enough that people walking in aren't suddenly crashing into those teleporting in. The point being that you put just as much security on the circle being used to get into the temple as you do on people just walking in off the street - nearly none at all, because the point is to get people into the chapel - it's all the
other parts of the temple that have locked doors.
Guilds: Another easy one;
teleportation circle goes in the commons or some other area that is designed as primarily public in function. Just like how the entrance from the street typically leads into a commons or a reception area, rather than straight into the workshop/vault/etc..
For other important places, I will assume they mean significantly large cities, and castles/keeps:
Large City: Think of it like an airport - it's not too far from where anyone coming in will need to be, but is nearer to things like hotels, restaurants, and entertainment than it is to things with higher security priorities. If the city has many walled-off districts, which is common in D&D city maps, then it isn't even that big of a hole in security to have a
teleportation circle in the middle of the market district or some grand plaza, because such an area can be open and in clear view of the folks tasked with shutting the gates out of that district if anything arrives that seems dangerous.
Castle or Keep: This is the sort of place where you probably mix physical security with a social element - you put the circle either in an open area like a courtyard or bailey that is under watch of guards, or you put it in an isolated locked chamber with guards positioned outside and the best lock you can afford on the door, and you keep the sigil sequence a secret only a few people share. That way anyone wanting to use the circle has to not only know the right sequence, they also have to have a key to the door (because the guards specifically do not have one as a security measure), and be on the list of persons the guards aren't meant to sound the alarm at the sight of.
And, as a bonus:
The party in one of my campaigns will very likely be making a
teleportation circle on their airship permanent, which will remain secure by way of only the party members knowing that the circle exists since it is located in the private apartment the party keeps on the ship, where the crew and any passengers or intruders cannot easily access (it being hard to get on the ship unnoticed, and then to get away from designated areas unnoticed, and then not be spotted by someone going about their own business while you deal with the
arcane locked door from the common area of the ship to the private area).