In the foreword to the map of Ankh Morpork, the writer tells a story about Pratchett saying the first question to ask when designing a fantasy city is "How does the clean water get in and sewage come out?"
What I have since realised is this also applies to designing aliens and fantasy creatures.
Mind you for a lot of human history the answer to the latter in real cities was either "people with shovels and barrows" or "into the drinking water", so...