The Temple of Hephaistos is one of two original buildings in the Ancient Agora in Athens. The other is the Church of the Holy Apostles. Hephaestus was the god of metal working, craftsmanship, and fire; essentially the god of engineers, and I find it somewhat amusing that the building dedicating the engineer is the one that survived!
Actually the building was used far beyond its original purpose, including as a Greek Orthodox Church, which partly explains its well-preserved state.
This church, which is found in the ancient Agora in Athens, dates to the 10th century. It is one of only 2 buildings from the Agora to survive intact.
Some of the original wall paintings still survive, and almost nobody goes into it. So if you want a few cool and quiet minutes in the busy Athens it is the perfect spot to spend a few solitary minutes.