This page is based on a section of User blog: Robin Patterson/Col1 FAQ by Toby Douglass. Comments on it should be on its talk page rather than on the blog.
Material copied from http://45mercystreet.com/computing/col94/index.html with specific (e-mail) permission of author (Toby Douglass)
The computer AI is not smart. It will attack a port colony and simply keep attacking that port colony. The simplest solution is to build a single port and have the other colonies inland from there, so they are not attacked; build a fortress in the port and stack it with cannon.
If you want to build up a considerable number of colonies, then there are deeper tactics to consider.
Cannon, the mainstay of defence, can move three tiles over road in one turn. When the Expeditionary Force arrives and attacks, you will find the east coast colonies come under considerable pressure and may well need reinforcements and replacements - sometimes urgently. At this point, having colonies behind the front line which are exactly two squares away from an attack colony (e.g. a cannon can move from colony B to colony A in one turn) is very advantageous.
The strongest formation of colonies is a square of four colonies. Each colony can move cannon in one turn to every other colony. Given the EF attacks from the east coast, there is quite a lot to be said for laying out a set of colonies in this formation (or a similarly strong formation - more on this later) with the sole aim of being well placed to resist the EF attack. Colonies further west can be laid out according to the tiles that are available, with a view to economic productivity.
Any colony placement scheme where colonies are more than two square apart (e.g. unable to move cannon between themselves in one turn) is fundamentally weak. Any colony which cannot receive reinforcements or replacements by the next turn is at risk. Consider; the EF arrive at say turn 200. Large numbers of cannon and cavalry land next to a colony which cannot receive next-turn reinforcements. They proceed to attack. At the end of the turn, you can see that colony is in danger - it has suffered losses and is still surrounded by hostiles forces. At this point, you now know reinforcements are needed. You could not know before this point. If the colony was within range of a neighbouring colony, you could move in reinforcements this turn and so save the day. However, since the colony is in fact say four squares from a neighbour, you can do nothing to support the colony until the turn after next - giving the EF one more turn to finish off your colony.
Weak colony placement is fine for inland colonies which will never be reached by the EF.