newb question: when to build an army?

The research tree is your friend!

The most serious type of attack in Illyriad is the siege.  Defending against sieges actually involves attacking the opposing army, which will be occupying a square outside your city.  So the advice on good armies for attacking actually also applies to defending against a siege.

Defending against a raid or attack on your city square depends on the type of terrain your city is built on and your race.  It also depends on the size of your opponent.  If the attacker is a fellow new player, your best bet is a good rune.  If the attacker is a much larger player, it doesn't really matter what units you have, since they are likely to be thoroughly crushed.  In many cases it is more prudent to simply move the army out of the way of a much larger attacking force, by sending it to attack an empty square during the time the attack will land.  The worst thing that will happen is that they'll take whatever basic resources are not protected by your vault.

Many larger players use bows, spears or a mix of bows and spears for defense.  Bows work best to defend mountains, and spears work better in forests.  Plains are hard to defend in general.

Note:  I haven't fought in an all-out war, so this advice is mostly applicable to the new player who is facing occasional random attacks, which is generally the most that will occur in Illyriad as it is today.

As discussed in my guide to self defense to new players, diplomatic attacks, particularly thieves, are in general much more of a concern to new players than are military attacks.  Therefore I recommend that new players focus on defending from this type of attack.

http://forum.illyriad.co.uk/basic-self-defense-for-new-players_topic2368.html

You want to think about what type of commander to use as well.  T2 cavalry commanders have the advantage of being pretty fast and having good attack characteristics, but you might not want to wait to fight things until you can build a level 15 barracks, which is required for t2 cavalry.

For a first commander, you might consider studying and leveling up Heroism and Forced March.  Heroism increases attack and will reduce your casualties.

Information on the choice of commander and which skills to level can be found in Fluffy's excellent guide.

http://forum.illyriad.co.uk/everything-you-need-to-know-about-commanders_topic2742.html