As well as improving the graphics of Island Commander, most of my time has been spent tweaking the AI. This involves playing the game and employing one of 3 basic tactics: build mines and wait for the AI to attack me; rushing the AI; and thirdly something between the two (build quite a few mines and then attack).
After playing it a few times, you begin to notice pitfalls that the AI falls into. For example, it doesn't keep any "reserve" cash, so when it gets attacked, it takes it longer to raise enough money for a defence. This has now been implemented. Also, it never builds any defences before it is actually attacked. I've still yet to add this.
There's also a few tricks that it can use that a human player would; for example, units are far more effective in pairs (or more) since they can get twice as much firepower out, destroying targets much faster. I've now implemented this.
I've also tweaked the balance a bit by adjusting various unit's stats; in particular, mines were a bit weak, meaning that the first player to attack usually managed to decimate the opponents buildings (causing the slippery slope effect). I've also reduced the amount of money that mines give, meaning that if a side does have less mines, they're not necessarily losing out on much more money than their opponent.
Thanks to all who have contacted me with problems and suggestions. It has made my job a lot easier!