Unlike most other shooters, Diadra is free-roaming. It's not total freedom however, the levels don't wrap-around like Defender. It's probably just as well because with all the bullets flying about it's liable to mess with your head. Before starting the game you can choose from different weapon sets. However, you have to unlock most of them through playing the game. Still, it's a nice to have something to look forward to on future play-throughs (aside from the whole "gotta beat my hi-score" stuff of course).
The controls are fairly easy to figure out. There's a strafe button, which is necessary so that you're not switching directions when targeting a large enemy. There's also a bomb. It can be useful for getting out of terrible situations, but I think it's used as a multiplier bonus trigger or some such. The dash button is one that you should get really comfortable with. Weaving in-between every bullet is fine, if you're crazy. The dash is a free pass through even the thickest enemy fire. Used sparingly you can slip through immense firepower, and deliver some punishment. If all else fails, you have a shield that recharges after taking a hit. However, with every shot the shield will take longer to recover. In-between stages a shop is available for upgrading weapon power and abilities.
Like any good 2D shooter, Diadra Empty is not the kinda game that can be figured out in a single play-through. The first stage is easy enough, with its tiny swarms of enemies that don't put up any resistance. In less than 5 minutes however you'll have more than enough trouble keeping track of everything. A handy radar points out monsters and even bullets, but presence of mind and knowledge of what will spawn where will bring you the most success. It gets extremely hectic, and I've seen so much crap getting thrown at me the frame-rate starts to buckle. Yeah I said frame-rate, there's no slowdown no matter how much is going on. The final boss is also one of those jerks with a dozen forms (including the obligatory "giant crystal" that you might remember from Radiant Silvergun, Ikaruga, and so on).
The skill ceiling for this game seems high enough. On my second attempt I managed to complete the game without continuing. That sounds impressive, but my score was absolute trash. I didn't even crack the default top three hi-scores. I definitely felt like I missed something during all the chaos. Also there's a "true" difficulty setting. Naturally this adds even more adversity to contend with. It'll be awhile before I give that a go, because I still have to figure out the scoring system.
This is a remarkable game, though it helps to be a huge fan of danmaku. Also unlike most titles in the genre, there's a handy maneuver for dodging bullets. Threading a thousand moving needles isn't required to grasp this game, which might make it more appealing than other shooters. Diadra Empty is going to hit Steam...sometime. You might want to give it a go when that time comes (or just grab it off Desura right now).