The castle is the place to which you go after each mission. There, you can restore the health of your hero and units, which can also be replenished there. This is the only place where you can spend gems and gold (although the latter is also needed for conjuring units to battlefield). There are three categories of buildings in the city.
Recruitment buildings - After buying a given building, you will be able to recruit new units. Each building grants access to one type of units. Some buildings also require another structure to be erected first - for example, in order to build the Mages Tower, you first need to build the Mages' Guild. The level of each building is represented by stars underneath its name (it can be increased with gems) and the number of units recruited (the blue effigy). Consider unlocking all buildings during the initial hours.
Special buildings - The Teleport (summons units to the battlefield), the Mages' Guild (store with scrolls), the Alchemists' Guild (store with potions) and the Training Chamber (hero's upgrades). Each of these buildings provides different advantages. Some should be bought as soon as possible (the Teleport); others can wait (the Alchemists' Guild).
Commercial buildings - The Jeweler can help you trade gems for gold and vice versa; the Imperial Market allows you to buy gems for real money. These buildings are erected from the very beginning and don't have to be upgraded.
What should I upgrade first?
Here's the order of importance of the upgrades we recommend:
- The Teleport. Without a doubt this is the most important building for your army. More units on the battlefield mean better tactical abilities and greater force. First levels cost only a single gem, so you should quickly upgrade the Teleport.
- Recruitment Buildings. The sooner you're able to assemble a diverse army, the better. Some missions will require only ranged units - in others, you won't be able to win without cavalry.
- The Hero. The skills that are the most valuable are: regeneration (replenishment of HP once every turn - just one level is enough at first), command (bigger units allow the soldiers to deal maximal damage for longer) and the increased number of potions and scrolls (also their range) available every turn. The rest is up to you - you can, for instance, increase the hero's personal stats. Additional gold is always nice, but you should have plenty of that for finishing missions.
- Military Upgrades. After a while, start upgrading your soldiers. Begin with the most useful, or those that you feel should be stronger. It's better to have a diversified army than just one, powerful type of units. Sooner or later you will encounter an opponent that will defy even your strongest unit - pikemen, for instance, are hell for cavalry.
- Scrolls and Potions. These are incredibly useful items, but upgrading the Alchemists' Guild at the initial stages of the game is pointless. There are loads of places where you can find interesting items. Only in time will these types of items become mandatory.