Found my way to Melindas Alley during a bar crawl and this was by far my favorite place we stopped at. Hidden away in the alley just north of the Renaissance Phoenix Downtown Hotel, you pass a huge mural and will see the guy checking ID's. Get your stamp and walk downstairs.
As soon as you walk in, you'll see a little lounge and you can look down the length of the bar. Red lights surround you, definitely adding to the aura of this being a discreet location. There are only two little bars to get drinks and the lines got pretty long in my experience with people lined against the wall. Drinks are pricey but delicious - and worth it because they are incredibly unique.
There isn't a ton of space so try and get a chair/table. When you're leaving, walk out the back which will take you to a set of stairs to take you up to the Renaissance lobby.