Sparta starts strong, with a very powerful army, and as such should not have too much of a problem conquering a few extra regions, with Argos foremost amongst them. Athens and you are constant rivals, so war is around the corner, but it won’t happen until a few turns have passed, as a peace treaty is running.
Be prudent though, as once you besiege Athens, the huge citizen garrison can make a sortie that can defeat you in the open field. Athens will want to drag out the war to amass enough gold for extra mercenaries, so don’t delay too long.
Sparta has a unique regional decision which enables the kingdom to levy an average quality troop, Perioikoi. They have a nice perk, in that they cost only one manpower in upkeep. This will prove handy to fill up your battle line with something other than Helots.
Beware of the Helots, they will revolt several times unless you maintain very high loyalty in Sparta itself.
If Athens is not allied with Sparta and neither is at war with Persia, then there will be a regular check to see who has the upper hand. This considers who has the most regions, the most legacy, the biggest fleet (at start there is no supremacy, but it can change very fast).
If one has supremacy over the other, then a reward will be given to the winner, and a penalty to the loser. Note that none of the rewards mean more legacy, extra ships or extra regions.
You can have up to 3 Helot revolt in Sparta (Laconia region), if your loyalty is under 75. The more slaves you have, the more chance of revolt, and this probability is only slightly reduced by stationing troops. It is nevertheless a good idea to do so, as the army will rack up XP quelling the revolt.
Sparta can receive up to five missions during the course of the game. There is no penalty in not doing them, but if you succeed, you’ll get extra legacy points and a special reward. Also note that if you fail a mission and if it is still valid, you might get it again in the future. Examples: Conquer Athens, Befriend Syracuse, Form the Peloponnesian league.
Sparta has one building at its disposal in the DLC, in addition to the 10 special buildings each Hellenic nation has. Note that Sparta has no access to the Ships Houses though (Neosokoi).
If used right, the stratopedon is very powerful. This is a field camp, that will only function if the region has no wall. It can give 5 XP per turn to troops, up to 20 units a turn!
The almighty but costly Spartan Hoplite will put a drain you all your resources, including your precious manpower. It also has a ‘Money Increase Cost’ of 20%, so you won’t have many in your army. To supplement it, you can draft numerous Perioikoi (but not in Laconia, see below) and round up the roster with Helots, which are cheap as dirt (and are worth about as much in battle).
Sparta, being a Hellenic nation, can use three regional decisions unique to this ethnicity. In addition to these, a fourth decision is unique to Sparta, Draft Perioikoi.
• Draft Perioikoi
Will draft between three and seven Perioikoi in the region, depending on the skills of the leader in charge. They will start without any XP if drafted from Laconia (this represents drafting the unruly Messenians) but can start with as much as 100 XP if you had a Military Expertise of 10 (10 XP per point).
• Implant Trade Settlements
Implant a trade colony in a coastal region controlled by an Independent nation. Yes, the region becomes yours! Note: Athens will receive this decision more than any other Hellenic nation.
• Panhellenic Games
Improve relations with the targeted Hellenic nation (even if at war). If your ruler is a good diplomat, the decision can even stop an ongoing war.
• Naval Support
Inflict damages on an enemy force adjacent to one of your fleet. A friendly army must be adjacent to this enemy force or in the same region (in case of a siege). Very handy to soften up the opposition just before an important battle. Contrary to Athens, Sparta doesn’t have access to the heavier ships until reaching Civilization Level III, so it will be harder to have enough combat power to use this decision.