
The way the treasure chests appear in Druaga is not random. You have to do certain things to make them appear. For instance, you may have to kill 3 green slimes in one stage, or kill 3 knights, etc. Some of the treasures you'll NEED later on, like the speed boots. It's a really obtuse game, but surprisingly popular in Japan for some reason.
Grobda is really more of a spinoff, not a sequel, to Xevious. There are these tanks in Xevious that just sit on the ground waiting for you to shoot them, and those are the Grobda tanks. Xevious is another surpristingly popular game in Japan, they even have novels with the Xevious backstory!