They might have a small tunnel as an entrance making it easy to defend, would probably still be more logical to have a narrow bridge over a natural chasm since it would make it harder to block. The chasm would also be useful to provide extra air circulation.

Even short surface tunnels would still be at least 2 people across and 6 feet high. The main problem with having a deeper tunnel be this small is your going to run out of air if you try to build a city with interconnecting tunnels.

Plus when you hit territory where canarys would be useful it's going to be at lethal levels for a much greater spread than if you had a wide area for the poisonous or explosive gas to disperse in. In your smaller tunnels you would probably kill everyone quite easily simply by cooking food.