But, for the time, these ideas were fresh and exciting. First person perspective was almost unheard of, as DOOM and it's legion of followers were over a decade away. This was realism as it had never been seen before for most players. Goblins, oozes, dragons... these were the same creatures that populated my favorite books and role-playing sessions, yet they were on the screen, controlled not by a Dungeon Master but by the computer itself. This was mind-blowing stuff for my poor preteen brain, and though I didn't know it at the time, Tunnels of Doom would be but the first in a long line of dungeon-crawling RPGs I would play through the following years.
You are probably wondering where the co-op comes in. To be honest, the cooperative elements in Tunnels of Doom were fairly light, especially compared to modern games. Four different players could control different characters, taking turns at the keyboard when it was their characters turn to act. The real teamwork came in coordinating the attack plan with one another. The fighters would move in to close range combat, while the rogue and the mage ran from direct confrontation to attack at a safe distance. That's a paradigm that has worked from SSI's famous Gold Box computer games, to Baldur's Gate, down to modern MMOs like World of Warcraft.
While only one player could control the party's movement in exploration mode, there was definitely a sense of cooperation when everyone made decisions as to the best direction in which to proceed. I didn't get to play Tunnels of Doom in co-op very often, sadly, as my friends and little brother had little patience for the rather lengthy dungeons. But it's clear to me now that the option of cooperation between multiple players was one more area in which the game was innovative.
My TI-99/4A served me very well for many years. By the time I was in high school in 1989, the poor unit had been used so much that it was literally falling apart, and I got even more fun out of it when I dismantled it before we threw it out. I've owned a computer of some sort ever since. For each system I've owned through the years, role playing games like Tunnels of Doom have been among my favorites. It may not look like much to modern eyes, but Tunnels of Doom got a lot right, and shares many features with modern RPGs. Not bad for a game that's going on thirty years old!