The Tower of Druaga

The Tower of Druaga (or 'TOD' for short) was released in June 1984.

This game was originally called 'The Tower of ANU' during the development.

Prince Gilgamesh is the same Gilgamesh of Sumerian legend.

Characters and much of the equipment found in the TOD series have appeared or been referenced to in many other Namco games.
"Tales of Phantasia" : Much of the equipment used by Gilgamesh could be obtained.
"Tales of Destiny" : Contained an optional dungeon based directly off of the original Tower of Druaga (but with different treasures).
"Tales of Symphonia" : The equipment used by Gilgamesh could be obtained, and the character Zelos Wilder could gain a special title by wearing it.
"Tales of the World - Narikiri Dungeon 3" : Also contains a variation of the dungeon. The game also contains costumes for Gilgamesh, Ki and Druaga which allows the heroes to become them in battle, and wearing the Gilgamesh costume is necessary to access the dungeon in the first place.
"Tales of Legendia" : One of the 'Rare Monsters' that you can find and battle is the Quox, the dragon enemies from the Druaga games.
"Mr. Driller - Drill Land" : Contains 5 worlds, one of which is called Hole of Druaga featuring a Mr. Driller game with RPG elements.
"Mr. Driller Ace" : Featured two levels named after Druaga characters - the 'Druaga Ruins' and the 'Quox Ruins'.
"Namco x Capcom" : Several characters from the series appear in this cross over.
"Baten Kaitos" : Eternal Wings and the Lost Ocean: The Tower of Druaga makes an appearance as a puzzle in Mira, complete with slimes and tools. Baten Kaitos also contains some of Gilgamesh's equipment as obtainable items.
"Soul Edge" : Gilgamesh's shield (the red-line shield) and the Blue Crystal Rod make appearances as alternate weapons for Sophitia.
"Soul Calibur II" : One of Sophitia's alternate costumes was based off of Ki's design. In addition, one of her weapon sets was the Blue Crystal Rod and Blue Line Shield, equipment formerly used by Gilgamesh and Ki. The Red Crystal Rod and Red Line Shield are also available as bonus weapons for Cassandra.

Namco released a boardgame based on this game (same name) in 1985 (in Japan only) : You move through each level of the dungeon to get a key. You roll a dice for movement, take the shortest route towards the key, and after every roll (the game does not even have rooms or such), you have an encounter with a monster that you must fight. You get a treasure for each victory, and once you have the key, you can open the door that leads to the next level, and continue doing so on each level until you get to the 6th level where the princess is.

Soundtrack releases :
The Best of Video Game Music [Alfa Records - 32XA-66 - April 25, 1986]