Borderline is one of several early arcade shoot-em-ups in the wake of Space Invaders, despite not taking place in space like most others. It's a distant precursor to Konami's Jackal and the Silkworm games, as you drive an armed jeep with the intention of infiltrating the enemy's base, and was developed for the Sega VIC Dual hardware. What enemy? Who knows, but they're the bad guys and you're the good guy. The first stage plays out as an action-racing hybrid where you drive across the countryside up to the enemy base's checkpoints. Your jeep is armed with twin cannons, but it shatters in a single hit as per conventional overhead shooter rules. Unlike other drivers and shooters, you actually turn to face horizontally instead of constantly pointing upward even while moving to the side. As you drive up, these obstacles which look somewhat like Japanese rice balls will jump up from their nestled position and attempt to crash into you. When you get near the end, you'll see enemy jeeps and tanks standing in your way. Behind them are cores, white objects which look like atomic nuclei, and destroying them is your objective in every level.
The second level slows things down and requires you to barge through what appears to be some sort of wheat field. For the next three stages, the game becomes less of a racer and more of a deadly maze game, where your objective is to avoid the enemies and destroy all of the white cores at the top of the screen. Third stage contains several solid walls, each with a fuel rocket on them that you can blast if this conflict goes on too long. This level is much like the second stage except without fields to drive through this time, so you'll have to make use of the walls instead to evade enemy pursuit.
The fourth and final area contains the base core, and it's not just guarded by jeeps and tanks this time. The core itself is surrounded by three walls, each of which transform into a twin cannon for a few seconds and shoot at intruders on sight. This perhaps counts as an incredibly early "boss character", as it takes more effort to defeat anything else you've seen so far. Should you succeed, you are told "NICE PLAY - GO ON" and are sent to the beginning of a new first stage with the wall and field colors noticeably different from last time. The walls in the first level are considerably tighter, and it'll take some skillful driving to reach the checkpoint without crashing. Later stages feature more aggressive enemies and increasing difficulty curves until you inevitably run out of lives.
Borderline was reasonably popular enough to warrant a Sega SG-1000 port, developed by Compile, and it plays almost identically to its arcade counterpart with a few altered nuances. It's almost the same graphically, though the sprites are a little less stretched out on the console screen. There's actually music playing this time, a sort of military tune that fits the theme and is actually less annoying than the loud beeps of the arcade original. The movements of the sprites are a little smoother and the enemy vehicles move a bit slower, but your own jeep is also a bit slower and baddies tend to shoot faster projectiles. The driving stage is made a bit harder with slightly tighter corridors and less time to move out of the way. It's also more difficult in some other respects (like avoiding walls while driving) but easier in others (slower enemy vehicles) and feels a bit better balanced for home play. The only negative is the lack of background color changes for each new round, so the appearance can get a bit dull after prolonged play. That aside, it's overall a solid port that might actually be preferable to the arcade version.
Borderline was also featured on the Sega Memorial Collection, Vol. 23 of the Sega Ages 2500 collection for the PlayStation 2, released in 2005. Included are both the original arcade version and a completely new remake. The original version is faithful to nearly a tee, outside of the slightly blurry graphics. The remake, however, is a completely different game, having been reinvented as a twin stick shooter. The player doesn't even control a jeep anymore, and instead guides a tank, like most of the other enemies.
The goal is to explore each stage and find the cores, which is one of the only thing it has in common with the original. The stages are not large, but require a tiny bit of exploration, and an onscreen radar helps you hunt the cores down. In the meantime, you're assaulted by enemy tanks, foot soldiers, and an assortment of turrets. You have a number of lives as well as a generous life meter. There are three weapons that can be switched between at any time - a machine gun, a laser and a flamethrower. The machine gun can be used as much as you like, but the others can only be used for a few seconds before overheating. The laser can also be used to destroy entire buildings, which is often required to reach some of the cores. The forced scrolling stages are completely gone.
The graphics are high resolution 2D, which are colorful but cheap looking. There's also a single orchestral-type piece of music that plays over and over. All in all, in spite of its unfaithfulness, it's an okay game, but it still grows tiresome after a few minutes.
Although not technically a port, a variation of Borderline was released by Sega in 1983 under the title Thunderground. The entire game is based roughly around the second stage of Borderline, where you dig through the dirt and destroy the six cores at the top of the screen. It's extremely simplified compared to the source game, mostly because the playing field is small, and there are only two enemy tanks on the screen. In later levels, only a single core is visible, and shooting one will reveal the next. The enemies also become more aggressive. Even though it keeps the jeeps/tanks theme from Borderline, the cover takes some creative liberties (as did many Atari titles) to represent the vehicles as drilling machines.