in the draft of my story, there is a city which is the capital of an empire. the empire has conquered many territories, but underneath the city, there is underground tunnels.

the enemies infiltrate and spy by using the secret tunnels, which the emperor and the governiment doesn't know about (only 1-2 spies at one time come in and come out of the tunnels and it occurs rarely).

in the final battle between the "rebels" and the emperor's government, the rebels will use the underground tunnel to infiltrate into the city and capture it.

the tunnels were built hundreds of years before by a different nation before the empire conquered that city and made it the capital (because of its location and lots of resources).

i'd like to know your guys' ideas on a "secret underground tunnel" and how plausible it would be for the emperor to not know about it and for how the rebels were able to thrive and infiltrate. thanks so much.


3 Answers 3



This is entirely plausible, and cities often only pay close attention to such things during sieges (when they're usually looking for NEW construction) or to monitor smuggling (and if there are no laws about taxation at the gates, this won't matter).

They're STILL finding hidden stuff under Rome - much of it buried without any note and forgotten almost immediately. You could have the entire basement of the prior kingdom's palace buried by conquerors trying to quickly erase the memory of the old government (Damnatio_memoriae). There's abundant good evidence of Egypt's Amarna period because the pharaohs that came after used the Amarna temples as fill - protecting the broken paintings inside walls.

The biggest risk is at the ends, or if there's a cave-in. But a previous emperor could have found it and kept it hidden as an escape route, or sealed the ends, forgot about it, and it was rediscovered decades later. Or people building a sub-basement broke into it and kept it secret for smuggling.

  • $\begingroup$ Not to mention sewers and storm drains. $\endgroup$
    – Mary
    Sep 15, 2021 at 2:51
  • $\begingroup$ this was a thorough answer that provided historical references thank you! ah, i see its the "ends of it" that are the issue- as im picturing the worldbuilding, i want the tunnels to extend from inside the palace grounds to outside the capital city walls and "end" at a forest that isnt frequently ventured. how would you suggest the "exit" and the "entrance" of the tunnel to look like? im having difficulty picturing it and i will do my own research of Rome and Egypt like looking at the references you gave :) $\endgroup$
    – user90189
    Sep 15, 2021 at 2:54
  • $\begingroup$ @Mary Or the forgotten sewers and storm drains (or underground aqueducts, or...). $\endgroup$
    – DWKraus
    Sep 15, 2021 at 2:54
  • 1
    $\begingroup$ @user90189 it doesn't take many decades for even a man-made concrete entrance to overgrow and become quite concealed. Modestly sized entranced carved by hand into rock are even easier to obscure. Tunnels could also easily link with or be adapted from existing underground watercourses, etc etc. $\endgroup$ Sep 15, 2021 at 8:31

One tunnel? Sure.

Put the exit into a place the infiltrators will hold, or a semi-public place they can infiltrate. The basement of a neutral merchant's home-office-warehouse, say, or the basement of the university. Brick it over until needed and put a jumble of old stuff there that is too good to throw away, to worn to use. Rickety chairs, chipped mugs, whatever.

In the best case an agent in place will secure and clear the exit. The merchant mentioned above, or the chief clerk of that merchant. A janitor for the university. Otherwise they clear it from the tunnel, more noisy but it should be possible.

The tunnel would only be found if one of the exits is found, or if new construction accidentally hits the tunnel. So go deep enough.

Many tunnels? More difficult.

With many tunnels, the odds of accidental discovery go up. Bad enough if the tunnels are not connected, since it would cause awareness of hidden tunnels and possibly a systematic search. If the tunnels are connected, the whole system is compromised.

Legitimate tunnels.

Why dig secret tunnels? The city has legitimate sewers, and all you need are secret bypasses for the places where those sewers pass under the walls. That's assuming there are big iron grates or something like that which are inspected periodically. The bypass has several doglegs and does not appear to go under the walls. It may or may not be bricked up.

Explain why they don't use the gates.

Not even a siege of a city that large will be spy-proof. If it is less than a siege, there will be thousands of legimitate travelers through the gates every day, and not even modern technology could make 100% forgery-proof papers.

What are criminal smugglers using? If they know the tunnels (or use similar ones) that could be problematic, but it would also reduce the outcry if one of yours is found. "Yet another whiskey-smugglers' tunnel. Tell customs and let them handle it."


The tunnels under the tunnels.

"It is the Crawler!" whispered Yanath.

"It's the thing I slashed on the stair," grunted Conan. "After it trailed us to the Eagle Door, it dragged itself here to die. How could the Xotalancas control such a brute?"

The Tecuhltli shivered and shook their heads.

"They brought it up from the black tunnels below the catacombs. They discovered secrets unknown to Tecuhltli."

Red Nails, Robert Howard

The emperor's people know about the tunnels built hundreds of years before. They do not know about what lies beneath the tunnels. This city has been occupied for 5000 years. By humans for 5000 years. It has been occupied considerably longer than that.

When the emperor's people occupied the city and set out to make a map, their local guides showed them a lot of tunnels. They made a fine map with lots of tunnels. There are other, older places. There are places where the local guides did not want to go, and did not mention.

Your rebels do not want to go there either, but they do.


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