The wonder Babylon (B) has a special ability on step 2: You can play your last (7th) card instead of discarding it.
These two questions cover if you'd have to play both cards in the 6th round or if you get a 7th round:
- Can a resource card be used more than once in a turn when allowed to build more than one structure?
- 7 Wonders question about Babylon guild
I wonder: Can you use this ability in all Ages (if built)? Or can you use it only in the Age you built the wonder?
I assumed that you can use it every Age, but then I noted that the special ability of Olympia (A), which can be used in all Ages, has "I, II, III" written on it. This clarification is missing on Bablyon's (B) second step, though.