1) What's the objective of the game?
The main objective of the game is occupying "The Temple" located at the center of the map. After which your kingdom can participate in Kingdom Wars once introduced.
2) How is the game world constructed? I see a list of Kingdoms that could resemble servers. Is there PvP across servers? How many players on each server, and how many servers/kingdom in the game?
Continents are servers imo, it'shard to tell, each continent as 6-7 Kingdoms. As of now there are about 112 Kingdoms total.
3) Are there seasons where everything is reset a la Diablo III, or is the game world perpetual?
Overall progress of a Kingdom are not reset, however there is an Event Cycle, some of these events are really good, others not so much. The main event of any Kingdom IMO is the The Mighty Governor Event, it's a weekly event with good rewards but you need to be on the Top 10 Overall so get the most out of it. There are also some minor events with great payouts.
4) I think I saw that the game advertises RTS gameplay but as far as I can tell you send commanders with units to do X and then they do, ie no actual user control of the fighting?
You can control the commander's movement whenever you want, if a fight isn't going your way, retreat or you can hold position. However everything else like using skills is auto. A commander uses his "Active skill" when his "Rage" fills up.
5) Is there a wiki or other source of information about units, strategy, tactics etc?
Wiki is incomplete and fairly new, so it's somewhat useless, YouTube on the other hand is a great choice for guides, tips and strategies.Some tips:
Join an alliance if you haven't done so already. Alliance members help and protect each other. They work together to secure various strategic points on the map, like Sanctums, Altars, Passes and Shrines.
If you are lucky enough to get a Legendary commander, always max out his 1st skill before upgrading his Star Level as their active skill is their strongest skill. My suggestion, keep them at level 1 until you max out their "active". If you are F2P focus only on one Legendary commander.
If you need Speed ups, buy them from Courier Station or VIP shop, they offer upto 70% discount.
VIP 6 is when you permanently obtain the 2nd builder, reaching VIP 6 is fairly easy in about 20 days for a F2P player.
NEVER ATTACK CITIES alone, only RALLY it, this is perhaps the best tip I can give you. If you attack a city your troops die, which means you wasted the resources and time to train them.
Join the Reddit page, you'll find faster response to your queries there.