Above all things, build roads first. That'll put some coin in your pocket and aren't that costly nor take very long to build. But the most important reason is that it'll increase your armies mobility, allowing you to do more with less. This way you can get by with just a single army covering the entire Iberian Peninsula. This, obviously, saves on army costs but has a nice fringe benefit too. All your battles will be fought by the same guys and their XP will rocket upwards as a result.