This mod adds small dungeons that serve as secret entrances to the cities in Oblivion. Perfect for sneaky characters, or if you've racked up a bounty and have to avoid the guards! Bruma and the Imperial City are not affected by this mod, as they already have alternative entrances the player can use.
Unique enemies can be found in some of the dungeons, they do not respawn and neither do any containers in the dungeons. Some vanilla nuisance-type enemies do respawn here and there.

EDIT: It was brought to my attention that NPCs were using the secret entrances, making them not so secret anymore. Because of this I've replaced all the doors with activators that look and function exactly the same as the doors. When activated they run a script that teleports the player in and out of the dungeons. Now the player should be the only one able to use the secret entrances.
EDIT2: As of the 1.3 update all doors leading in and out of the secret entrances have been marked as "Minimal Use" and "Hidden". This means that NPCs will not use the doors regularly but are still able to use them in certain situations, for example when following the player in/out. Thanks to LenaWolfBravil for pointing this method out. I've also tweaked the mod a little, mainly by adding some flora around the Cheydinhal and Leyawiin exits.

The entrances are:
Lorgren's Forgotten Tunnel leads players in and out of Anvil. Lorgren's tunnel key can be found in the basement in Benirus Manor. A trap door behind Benirus Manor leads into the tunnel and exits into the sea behind Castle Anvil.

Lost Smugglers Grotto leads players in and out of Bravil. There are two entrances, one in the water next to the docks and another one on the castle grounds. The exit is in the shallows behind Bravil.

The Abandoned Basement leads players in and out of Cheydinhal. The entrance is located behind the Abandoned House. The exit is hidden beneath some burned ruins, circa northeast of Harlun's Watch.

Emero Av Cey leads players in and out of Chorrol. A trap door behind Castle Chorrol leads into the ancient ruin. The exit is in the ground just above the fork in the road above Weynon Priory.

The Old Leyawiin Dungeon leads in and out of Leyawiin. The entrance is in Alval Uvani's backyard. The exit is located in a ruined tower behind Leyawiin.

Glarthir's Secret Tunnel (built by Glarthir just in case) can take you in and out of Skingrad. Glarthir's secret key can open any locked doors and containers here. The entrance is located in Glarthir's basement and the exit is hidden amongst some rocks outside Skingrad.

Compatibility
This mod will most likely conflict with any mods that change the city layouts.
Might run into problems with Unique Landscapes.
Makes slight edits to Glarthir's basement and Benirus Manor basement.
This mod has been cleaned with Tes4Edit QuickAutoClean.