City Resource Buildings

Leveling  you castle unlocks more plots of land (look at castle "info" screen for details) for resource buildings (food/wood/iron/stone), medic tents and training grounds, up to 42 total - use these slots as follows:

1. Medic Tents - you absolutely need the max of these to be able to heal a large army after an attack. Build 8 of the possible 8 as more are unlocked. You don't need to upgrade these aggressively in the first few days of a new account, but as soon as your army size begins to stretch your hospital capacity, build and upgrade these aggressively.
2. Training Grounds - let you recruit more troops with a single action - I recommend maxing these out (8 of possible 8) as well so that you can do long overnight recruits when you unlock higher level troops
3. Food & Wood - I'd build 6-8 out of the possible 10 of each of these, saving some slots for other building types. This makes the early part of the game (before you need much stone, iron) easier for newer players.
4. Stone - I'd build 6-8 of possible 10. These unlock at level 10. At first you'll have modest stone needs and it will pile up. Make sure you save one spot of land to build at least one as soon as they unlock - having a stone mine is a pre-requisite to further cast upgrades. You can sacrifice 2 wood or good to max these out if you want to reap late game benefits (you need a ton of stone and iron for big castle and other upgrades).
5. Iron - I'd build 4-8 of possible 10 (more unlock at higher levels - most players miss this because their lots are already full so they lack the option to build more). Unlocks at level 15. When your castle level is 16, you'll find that you don't use much iron. When you start passing 18, you'll likely be running out quickly. As with stone, you can sacrifice 2 food or wood to get the maximum 10.

FARM ACCOUNTS - For your farm castles, build one training ground (can't upgrade your castle without one) maximum of all resource sites - 10 farms, 10 wood, 10 stone, 10 iron totals 41 - then 1 medic tents - you need at least one to upgrade your castle - to get to maximum 42. If you're going for a "strong farm" (to kill monsters for xp, or some other well thought out purposes), build a couple extra medic tents at the expense of some resource buildings

General resource building tips/considerations:

• Wood and Food are easier to pillage than stone and especially iron - most people who quit and end up being farms do not have high level stone/iron resources for you to take
• through around level 13, these upgrades are cheap and fast. Beyond that they begin to eat up considerable resources, so you should defer picking these up if it delays upgrading the higher impact buildings - castles, drill grounds, barracks, college, etc.
• Pick one of each of food/wood/stone/iron buildings to upgrade to highest available level, as the are pre-requisites for castle, hospital, watchtower, etc. upgrades (only one, not all, has to reach the specified level)
• Your fields can hold a maximum of around 12 hours of production, so harvest these often. Unharvested resources are always susceptible to pillage and are not protected by your depot level.
• If you have a change of plans, you can destroy one resource building to make room for another - if you're not sure about a resource building, don't upgrade it past level 10 (low return on resources invested - at higher levels these take a long time to pay you back) so that you don't end up wasting too much.