Mountain Citadel vs. Dark Citadel

[Maltrease]

The wizard tower and castle do not have bad ROI's, particularly if playing on a Rich map setting.

If you built a wizard tower in a provence generating 100 income it will increase your income by 15 gold a turn, and pay for itself in 20 turns.

Finding provences with higher base income produces even quicker returns. Obviously if you plop a fortress down on a provence generating 20 income it will never pay for itself.

If you playing with high income scales (3 order, 3 production, 3 growth), rich maps, or big long games this can a significant motivation in your plans.

It also combines nicely with the lvl9 nature enchantment spells that doubles your income.

And of course having lots of fortresses has many other benefits besides simply enhancing your economy.