I've had 3 HUDs throughout the life my game due to poor planning, here are my lessons learned:
My first HUD one was very cartoony, and just didn't match the look and feel of my game. Make sure your HUD graphics match the theme of what you want your game to be.
My second attempt was more fitting for the style of the game, but took over way too much of the game's screen real estate. Don't dedicate much screen real-estate to your HUD. Making it translucent can be nice too in some cases so you can still get some view of the obscured areas.
My current HUD is very simplistic and designed to obscure as little screen real estate as possible. Light translucency is used so you can always still see what's behind the HUD.
Use animated touches when things in the HUD change. For instance in my game the new weapon icon appears larger at first and shrinks back to normal size over about 1 second. Somehow this small effect makes the weapon changing experience much better. Health meters don't instantly change, but rather the lost health turns yellow and shrinks at a linear rate. While a city builder has neither health nor weapons, I'm sure there will be parts of your HUD that can be enhanced with a simple animation.
I've attached a screenshot showing the 3 HUDs