Strand of the Ancients Defense is unique among battlegrounds in that your team is free to move on the map during the initial countdown to the start of the round. A common mistake by less experienced players is that they stand around at the initial spawn point. The defense absolutely must use the time to get in to position to engage the enemy as soon as they land on the beach.
Right at the start of the game, you want to have people on the turrets for both the blue and the green gates. The rest of the group goes onto the beach getting ready to greet the incoming offense. Defense should be relatively split between the two landing docks, as the offense will also be split evenly on the incoming boats.
The demolishers have to be destroyed as soon as possible to prevent the gates from going down. Therefore, the one demolisher on the beach is often popular to destroy first. Demolishers also take priority to kill over players, unless they were hindering your ability to attack the demolisher. If you can crowd control any players trying to intercept your assault on the demolishers, then do so and continue on the demolisher until it is destroyed. Upon destruction, the driver will dismount and be very vulnerable if caught in a large group that just destroyed the demolisher.
Once the first gate is down (blue or green) the Eastern and Western graveyards are now open for the enemy to take. As soon as these graveyards are taken, the enemy gets better forward position when resurrecting, and additional demolishers from the adjacent workshop. Though it is still more important to destroy demolishers and keep the enemy from breaking the next gate (red or purple), holding these graveyards will do a lot to hinder the advance.
Pay attention to your map as well. Just because you're holding off people just fine at the purple gate might not mean the same for red. All active demolishers are visible on the map to all players. A single demolisher that has reached a gate with no one nearby to destroy it will be able to burn through a gate in about 15 seconds. Be prepared to concede that gate and fall back to the next one.
If there are demolishers incoming to the yellow gate, you will have to fall back and destroy it before trying to push forward again on defense. Typically, once both red and purple gates are down, the entire defense should fall back to the yellow gate and be prepared to destroy vehicles coming from either side. Watch the map to see where the closest demolisher is, and move towards it as a group. Do not wait for it to get within firing range of the yellow gate, or it will eventually go down. Good teams will send demolishers in groups of two from each vehicle emporium, which makes it more important to intercept them early and burn them down before they reach firing range of the yellow gate. Always watch the map and be ready to mobilize and meet demolishers coming from either side.
If the yellow gate goes down, you will have to use it as a chokepoint to prevent players and demolishers from getting past. If your entire group has not moved back and tanks are not going down quick enough before being able to touch the door, you will not have the best chance of lasting the timer.
While in the final courtyard, a few players on the offensive side tend to stray off and go west towards a pile of seaforium charges. If they are not intercepted, they will circumvent your group to the door and plant the seaforium charge. A couple people with good crowd control can guard the steps and prevent a seaforuim assault on the final gate. Though, it is best for most of the team to be at the yellow gate chokepoint, to prevent players from ever getting in the courtyard, and to burn down demolishers as quickly as possible.
If the door to the keep goes down, do your best to drag out the timer for as long as possible by keeping players out of range of the orb either through snares, disorients, stuns, or knockbacks. Any player on the opposing team has to be just lucky enough to charge through and click on the orb in order for the offensive team to win the round.
The most important thing to remember on defense is to hold back anything that can do damage to the gates. This means always attack demolishers as soon as possible, before they are in range of a gate. Also be on the watch for players who rush past towards a gate, as they probably have a seaforium charge to plant. Always work to push the offensive assault back, but be prepared to fall back if something slips through. All it takes is one uncontested demolisher to roll through every gate and lose the match.