Having a cannon engineer get on the cannon at your castle is a highly effective way to defend your castle when you have someone to protect them. Have them fire on the bridges as people try and pass and they will simply be destroyed. Meanwhile you can use the same strategy on the opposing teams bridge. If you get your team to block them off at the choke point and protect an engineer while he builds a cannon you can take control of the bridge with dominance.
Once the bridge is secured you launch team skills from behind this point in order to pass through the choke point and capture their home tower. Keep in mind that there is a collapsible bridge off to the right that must be guarded as well. Once you take control of the enemies home tower then have your team defend it as mages launch cyclone on the bridge and chase anyone who attempts to pass.
At this point the cleric from out castle would push forward and set up a cannon just past our base so that if the game does come down to time they can fire on the center It is important to deceive the enemy, leave that cannon with your engineer and push forward so that no one targets it before the time gets low. Instead push forward and go help with team skills, or build a cannon at their base, or build a gatling gun on the ledge at their castle right after the collapsible bridge to fire on the gate. Or simply cast rush plus on the mages who are chasing people and hel them with defense.
The cannon firing on the enemy bridge will prevent them from getting past, having someone protect the collapsible bridge aswell is also important so the the mages can remain protecting these points instead of chasing players all the way back to our base. If reinforcements are needed at our base then call someone back so that they do not do any damage or build anything.
The only weakness of this strategy is the it clusters the enemies into one location so it is important to use as many team skills as possible and stick close together. If you spread yourselves thin then you will not kill them fast enough as they re-spawn and they will overwhelm you simply based on numbers. However if you bait them by forcing them to defend their gate and have players kill them as they focus on the gate them you can kill them fast enough so that they are few as they re-spawn and your team skills will be interrupted less. The mages on defense can push ahead and take out the enemies gatling guns on the wall and clerics can build gatling guns of cannons by their castle.
Of course if the time gets low then everyone must retreat to the center and launch team skills there. Some must also go to our castle and defend our home tower. A cleric will be on the cannon defending the center so have a player defend him as well because the cannon is key.