From ErfWiki

Tunnels are a City Zone bordering the Dungeon zone of a city. Tunnels are also a Terrain Type

Proposed Canon

Tunnels can only be entered by light Units or units with a natural ability allowing them to enter tunnels, like Spidews and Sourmanders. Tunnels can attack, or be attacked by, the dungeon zone of the Garrison Zone.

Speculation

Tower may be a zone in other cities, or even other terrain, as well. Each system of tunnels may have unique rules based on its own construction. The restriction that only light Units can enter Tunnels may hold for all tunnels.