Golgo 13: Top Secret Episode

Unused Area
In Act 3, there's an optional building, which contains a corridor. The corridor cannot be passed, as you need four grenades to get through and the game only provides 3. However, if you use a cheating device to get through (Memory $51F = grenade count), you will reach a down ladder to the second basement level.

The second basement (location identifier $5B) consists of one-way walls. There's nothing else of note, and you can safely return to the rest of the building by returning to the start of the floor.

Additionally, using a memory editor to move outside the 4x4 area may let you stumble upon a down ladder. Taking any of these ladders brings you to the beginning of this floor (as if you entered from the second floor.)

Level Select
To enable this cheat you must wait until the helicopter demo ends, and wait untli Duke's eyes show up. Press Start to skip to the title screen before the text appears. At the title screen press and hold Up + A + B on the first controller and Up + Left + A + B on the second controller. Now press Start on the first controller. A screen with two numbers will appear. Press Up / Down to select stage code. Codes of the levels are:
 * - Level 1
 * - Level 1a
 * - Level 2
 * - Level 3
 * - Level 3a
 * - Level 4
 * - Level 4a
 * - Level 5
 * - Level 7
 * - Level 8
 * - Level 9
 * - Level 10
 * - Level 13

Regional Differences
In Stage 2, Golgo goes to a hotel room to rendezvous with Cherry, she ends up offering herself to Golgo. In the USA version, that's where it ends. But, in the Japanese version, it actually shows Cherry taking her clothes off first!