The Emperor's Castle is the best place to make money and gain experience. It has vendors in there, as well as about half dozen places to "save game". The monsters in there have minimum 50-60K HP, and hit fairly hard. So you have to be able to at least do 10-20K damage per hit when you get there. You can gain levels very quickly in there (almost a level every 2-3 fights), so your HP is set back to full very often. My character is clvl 94 and doing over 330K damage with the fire sword (base 8000 damage). Good luck!
I had to stop at level 23 and do something else. It won't let me continue after I came back. The level select works but up until level 23. I have to play 23 levels all over again!? That is the definition of lame! 1/5
The secret to the game (not much of a secret actually):
Shield: slvl 99
Energy: slvl 99
Attack: slvl 99
Gatling Gun: slvl99
Ship: Odin
Weapon: Excalibur
I used the "Heimdall" defense oriented ship (awarded after level 5 I think) until I got Odin, which was for most of the levels. I kept upgrading to the different homing-missile weapons, which is a mistake. Keep to the Gatling-gun weapons.
The game is all about replaying the levels until you get all your skills up. It's very repetitive. If it wasn't for the badges, I probably wouldn't have bothered.