If you're focusing on magic towns is usually a bad idea, you want to go with conclaves to max out mana income and research.

Generally focus on conclaves, but to get higher-level cities I'm going to guess you'll need at least one or two towns with those buildings. Plus, gold is always helpful for rushing buildings.

I tend to build Conclaves in all high essence areas, and Towns/Fortresses in all non essence areas.

I usually build one town on a low/no-essence settlement, one fortress in a high-production spot that ideally has 2 essence, and have most of my other settlements become conclaves.  I do like to keep things balanced to satisfy my neuroses, but I generally lean conclave-heavy.

I tend to non essence = towns, essence = conclaves and one fortress on a reasonable material two plus essence site to make troops.

I may throw down a fortress in a good choke point or on a boundary I am worried about but generally find that by the time they become any good the boundary has moved on.

I like towns for their nation wide food increases and their bonus to unit hit points.

Nakisisa