This is a very strange game but it's also incredibly addictive. Having this with you all the time is just the best way of killing time ever invented, as you don't need to spend very long on a particular battle (they usually have timelimits of 30 to 90 seconds) but you can spend hours at a time upgrading your units and wandering around the landscape finding new armies to beat.You upgrade your units a bit like Pokemon, they evolve into their new forms, but unlike Pokemon experience isn't enough, they only evolve when they're fed the right Emblem. You create Emblems by combining items you find in the world (such as jewels, leaves, metals etc) and experimenting and finding the right recipes to make your units evolve is one of the key parts of the gameplay.
Pocket Kingdom is Massively Multiplayer Online which means thousands of real people all around the world control armies wandering about the place, and (thanks to the n-gage's mobile internet connection) you can play against them anywhere which is especially good on the bus, on a train etc.
The world rankings of all the players is displayed on the N-gage Arena service (which is like the N-gage's equivalent of Xbox Live). There's also a built-in special version of the Arena Launcher which lets you read and post to the Arena message boards, chat to other players, read news and hints etc.
Sega worked on this for 2 years and the whole thing feels wonderfully unique... it's definitely the best portable game Sega has ever done.
The one drawback is the menu system which can sometimes be a bit frustrating (you have to buy items individually rather than being able to buy them in bulk for example), and getting started on the Emblems is a bit too difficult. Here's the first one to start you off: Warrior Emblem = Rankup Mold, Copper, Garnet and Black Leaf
I'd take off half a star if I could for the fiddly menus, but as it is there's so much good stuff in the rest of the game I'm leaving it as 5 stars: one of the best portable games ever.