You could also read my post about defending in the neighbour thread and reverse it Push pipes with shieldbros only, push balcony only under assault marines cover (well, they will die but you will advance), dont be a coward, assault is simplier in terms of tactics for now, because you dont have to count lives.

They are juwt using classic chaos tactics. Infiltrate, destabalize, and bring down from within. The best siege work starts well before a siege even begins

This one recognizes the truth, and if it's tips you want...

Cover vindicators. This is a priority. Kills don't matter.

The whole essence of the fortress assault is making sure you bring down the walls as early as possible, leaving the wall up basically means you are hard pressed against havocs by trying to push kill zones on foot... very ill advised.

This can be done with the first spawn of vindicators and takes no more than 5 people in a squad/war party;

1. Take vindicator to left side and park it inside the ruins in front of the left wall, you can fire from inside (next to the stairs) and still hit the gate and will be covered from the majority of incoming fire.
2. Ensure your 4 team-mates are in a rhino following you up, parking it behind the rock to the right of these ruins, ensure that you are all havocs, 2 heavy bolters/2 lascannons and at least a repair kit or two.
3. Lascannons will melt the left most quad (the only one able to hit the vindicator if parked right) while the HB's suppress enemy havocs on the wall that are gunning for the vindicator. The lascannons also suppress enemy fire by themselves, even if your red death beam is forcing the defenders to move around, it means less attention on your tanks.
4. Plenty of team mates will have followed you up to the left by now creating a delicious distraction for the defenders, just focus on the gate and keeping your ordnance alive.
5. Left gate down.

Follow suit with the right gate, there are more ruins to the right of the bridge to park under but since you tend to have more enemy havocs there (and possibly a lack of tactical thinking...), your first vindicator may not be successful, bring the left vindicator over with the same squad either by rhino or on foot (driving back behind the ruins) and finish it off.

Remember, once a gate is down the enemy cannot spawn on the wall and it will be easier to clear out with more specialized assault classes. Once you have the wall (assuming you have done it on first vindicator spawns), it is a simple case of using the position to your advantage to shower A with plasma and bolters. Assaults can charge in sure but to be honest, it is far more advisable to whittle down their tickets and force them to retreat back when they reach low figures.

You have plenty of time if those gates are down early, PLENTY, do not press them back into B early or you will be faced with 150+ tickets turtling an extremely defensive position.

You'll have lots of lone wolves on the team, lots of people mindlessly charging the stairs only to get melted by the defenders, use these valor thirsty baboons to your advantage as cover for the real prize. Only takes a squad or less to use their brains and they can calculatingly tear the defense apart with Iron Warrior efficiency.

Of course above is the desirable scenario, and a good defensive force will send in jet assaults after your tanks if Havocs can't find the mark. Adapt, if the defenders manage to keep both gates up after 2 vindicator spawns then you're doing it wrong.

Good luck.

I have tried this only once,
Take both vindicators to the left and destroy the left gate using both vindicators. Then move them to a location to repair to max using a rhino for UNLIMITED REPAIRS!!! and repeat the double destruction on the main gate.

Is that...a picture of burning trash?

Found my new desktop pic.