Creating a successful 'freemium' game is an exercise in enticement and restraint. You need to slowly but surely gain the player's interest, building excitement to a fever pitch before introducing the idea of laying out a little cash to enhance their enjoyment.

Or you could just give the player seven levels and demand they pay up.

The latter seems to be the path Chillingo chose to take with Spice Invaders, a rather entertaining and polished little defense game that's packed with plenty of features if you're willing to pay the price. Space Pirates have come to Earth seeking our valuable spice, and your job is to secure your base until Chillingo can come along and ask for money.

A relatively simple example of the defense genre, Spice Invaders sets the enemy loose on a series of colorful playfields, the player tasked with placing their defense in such a way as to control the flow of bad guys heading for the base. It's about creating paths, and then littering those paths with destructive elements to ensure the enemy doesn't reach the end. Each of the game's 55 levels features multiple modes of play, including a couple of online multiplayer modes, perfect for getting defensive with your friends.

When the player triumphs they gain levels, each granting them points that can be used to upgrade statistics and skills and a certain amount of the titular spice, used to unlock new weapon upgrades.

As with any good freemium game, spice can also be purchased for real money, with $10 or so earning enough to uncover much of what the game's limited tech tree has to offer. I've no problem with this sort of freemium.

The issue is that you're restricted to the first seven levels until you either reach level 20 (not an easy task) or pay up 42 spice. Seeing as most players will be heavily investing their spice towards upgrading weapons, this essentially means it's time to pay up.

Purchasing 200 spice only runs $2.99, so it isn't an issue of cost; I would have gladly paid $2.99 for an unlocked version of the app in the first place.

No, this is an issue of having the wind knocked out of your sails. Just when you really start getting into the game your momentum is sapped by this artificial barrier. It's not a large barrier, but smacking into it is jarring enough that suddenly I don't feel like playing anymore.

Which is a pity, because Spice Invaders is quite a polished little product, mixing simple game mechanics with a whimsical style to create a game I'd probably still be playing right now if I didn't feel so sucker punched.

Spice Invaders [iTunes]