Another simple and straightforward stage. The only thing really worth mentioning in the level itself is the fact that there are two branching paths, the top of which can be reached with the Rush Jet.
The top path is ridiculously easy, while the bottom one is a bit more challenging and features the cowboy robots from Tomahawk Man's level. Take the high road and dodge the tough stuff.
The last hall of the fortress is a bit of a challenger, with plenty of bottomless pits and robots that want to knock you in there. Stay on your toes and use the Rush Jet often.
Mr. X himself fights you in a machine on a chain, that drops various shots to hit you wish. Go nuts and melt his circuitry with the Flame Blast. Before long, you'll discover Mr. X's secret (if it wasn't already blatantly obvious.)
Onward to the second fortress.