Question: Is a curved Menorah kosher for Hanukkah?
Answer: A round Hanukkiyah, even if the candles are not exactly at the same level, is kosher.
The ninth candle - the Shammash - should be a significantly higher level because it isn't counted into the eight. The purpose of the Shammash is to provide light during the burning of the candles, just in case someone accidentally forgets and uses the light of the Hanukkah Menorah for a secular purpose, e.g. sewing or reading.
The ruling that a curved Hanukkah Menorah is kosher is based upon a teaching of Rava that recalls when a Hanukkah lamp could have had oil and multiple wicks. So long as the bowl of oil was covered with separate openings for each wick, then one could use the one lamp. Such a lamp would clearly be circular.