Here are tips found online that may be useful to you. It talks about strategy. That being said you may find other strategies useful.
Think of this as a starting point on how to win. Good luck out there!
Communicate with your teammates. Much more than Conquest, Stronghold requires reactively changing to adapt to the opponent's strategy. If you're by yourself on defense, and you see the enemy is doing a full team rush, let your team know you see 5 incoming, do a map ping when you know you can't defend the gate / lord with what you have.
As a general rule, be nice, no name-calling etc., but this should be obvious. Your team is more inclined to help you if you ask nicely instead of calling them names for having 3 members on offence (and you only call them that after you say absolutely nothing against the opening plan).
Until it gets to lord battles, enemy players are always secondary. I see so many people waste time trying to fight each other instead of focusing on the objectives. Unless you're defending your lord, there is always something more important to do, whether you're on offence or defense. If they're really pestering you, use control skills on them.
The opening split is important. The 3 major things you need at the start of the match is offence, defense, and supply.
A typical split is 2 offence, 2 defense and 1 supply. Somewhat counter-intuitively, defense should have a lot of crowd control and burst, while offence should have a lot of sustain and support. Supply run should be anyone who can move quickly and possibly stealthily. Supply is a roaming task and should be ready to switch to offence or defense at the drop of a dime. 3 offence and 2 defense is also a solid split if you're confident that you don't need the extra door breakers.
Be ready to react. You'll need more on defense if the enemy focuses hard on their offence. Likewise, more on supply and offence if they are focusing hard on defense. Communicate to your team how much resistance you're getting as soon as possible.
When your gates are still up:
Kill enemy door breakers ASAP! Enemy players can't hurt the gate at all, and Archers and Mist Champions do some amount, but door breakers are the most damaging of all to gates.
Keeping your gates up is the most important thing in the match for defense, so knocking out or controlling door breakers is absolutely vital. This buys your offence team much more time than if you were trying to go for enemy players.
Use the trebuchet if you're good at aiming it, as it two-shots door breakers and they often cluster in a group.
Protect your guards. Guards can one-shot enemy door breakers, so they serve as a vital defensive point. This is secondary to killing door breakers only because it is immensely difficult to protect guards if the enemy offence team is large and/or skilled. If you need to make a choice, go for enemy door breakers over protecting your own guards.
When both your gates are down:
Sustain and bunker on your lord and your NPC allies. Naturally, lord is highest priority because if they go down you lose. Your lord and NPC guards are also rather powerful and can deal quite a bit of damage against any enemies attacking, so you can freely focus on team support and team healing and not worry so much about attacking.
Kill enemy players. They tend to trickle in one-by-one after a failed first assault. Take advantage of this, as every kill nets you 5 points and if the timer runs out, enemy kills really count.
If there is someone else on your team babysitting the lord, hamper any enemy Mist champion that the opposing team may have summoned. They have powerful aura effects, but you have a prime opportunity to hurt them as they're walking up the path, due to the fact they don't fight back until they reach your base.
Note that they have a break bar so immobilize and stun options aren't recommended unless you're confident you can break it.
If a Mist champion reaches your base at full health, it is a much harder fight if enemy players are with them as well.
The trebuchet still works all right in this stage of the game if you're good at it, as you can destroy archers and weaken Mist Champions with it too before they reach the lord room.
When enemy gates are still up:
Kill enemy guards. This is priority over protecting your door breakers, because these guys one-shot your door breakers and there's no way to defend them from that. Plus, it's pretty easy to kill them, even if there are enemy players trying to protect them. They're the biggest hamper to the offensive force.
Protect your door breakers. If the enemy is using a treb on you, scare them off of it, but don't waste too much time destroying it as it auto repairs over time. So long as an enemy isn't actively trying to use the treb, keep off of it and instead, heal and boon share on your door breakers while controlling enemy players trying to kill them.
When enemy gates are down:
Focus fire lord, and use any defensive skills you may have like projectile protection or reflects so that enemy defense will hurt you less. Use lots of AoEs to try and catch enemy players or NPC guards in the crossfire. Break the enemy lord bar if you can to stop them from using their effects. Save your invulnerability and/or stability skills for when you can stomp the lord. Run away if you're going to die, heal up and then return to the fight because you're always better alive than dead.
If your initial assault failed, do NOT go in by yourself after respawn. If your team consistently trickles in one at a time, all you do is feed enemy team points. Make sure you have at least 1 other person with you when you make the next assault. Optimally, enemy defense count + 1 person should be on the attack force, if you can spare it.
If the enemy team has 3 or more defense on lord (or their defense is just really strong against your attacking forces), but you are up in points, join the defense or supply / Mist champ roaming team and force their hand. They'll need to risk making assaults on your base, thereby giving you more points from more kills and you can just stall the timer out.
Supply
Probably the easiest job, but nonetheless critical.
Do it stealthily and avoid engaging enemies if possible. If there are lot of enemies guarding supply at the start, let your team know so you can re-appropriate forces for a moment.
Door breakers while gates are up, archers when gates are down. Pretty simple, really. Archers are lower priority than Mist Champions, however.
MIST CHAMPIONS. There will generally be 1 enemy player going for it as well. Do your best to win the 1v1 to get the champion. If you're lucky (yes lucky), there will be 2 enemies or more going for the same commune point. In that instance, dance around, pretend you're going to try for the commune, but in actuality, distract them for as long as you can away from doing either their team offence or defense. Don't fight too hard, just sustain, and don't be afraid to run away if things get too difficult.
If your offensive team is wrecking gates and guards no problem, stop your supply run and help with the defense. You typically only need 2-3 extra door breakers over the initial 5 if the enemy defense isn't very good and your offence is decent. In the event both gates are down, you can join the attack as well if the defense is holding up decently. Reacting properly here is crucial.
And that is Stronghold strategy in a nutshell.
Door breakers take 2 treb hits to kill. Not sure on the archers, they're probably the same.
Treb can also be useful in targeting the Mist Champion commune points. Of course, they're also useful in disrupting the supply pick up area as well.
The opening split is important but you dont need anyone on supply at the opening. you already have 5 doorbreakers just protect the ones you have instead of wasting time summoning new ones.
Once the first gate goes down, unless you have some crazy burst and control, the enemy guards will kill a few of them. If the enemy had someone on defence, it can potentially be all of them.
Typically, 2 people on offence is more than enough to protect the initial 5 all the way through to busting the inner gate if the enemy defence poses no threat.
Having a third there does not speed anything up. The extra supply at the start serves as an insurance policy in the event the enemy defense was stronger than expected, and it can make quite a difference. Even getting one door breaker to the inner gate can be enough, but if there were 2 or 3 enemies on defense, that door breaker you save to get to the last gate is likely never going to be from the initial force of 5.
Mist champion is only interruptible by stuns, dazes, knockbacks, etc. Good strategy is to either pop stability at the start because most people will try to interrupt you add hard as possible.
Literally won every game by going for supplies at start without building door breakers.
2 people with mobility go for center, stealth grab supps, build 4 archers. rush doors. win.. pew pew.. easy.