This scenario is impossible to finish with a Crusader's best friend: moats.
Although the goals are economic, success depends on surviving two massive attacks. So forget baking bread and hoarding cash, focus on an archer and ballista based military industrial complex.
At its peak, I had 15 fletchers working.
To start though, I demolished every wall and tower, including the annoying staircase-up, staircase down structure separating the keep from resources in the east. Doing this greatly speeded production.
Lumber is the limiting factor here, so I built 20+ woodsmen to work the SE forest AQAP. After that, it's hops farming to generate the booze required to pacify the starving masses. (Wheat farming consumes too much wood to be feasible.)
Then its quarries, fletchers, and iron mines.
I also built a cathedral and smaller church immediately to generate a +3 popularity bonus to pacify my starving hordes.
But the real key to winning is moat digging.
In the SW, I demolished the staircase wall, dug a moat and built a gatehouse to prevent incursion from this direction.
In the south, I dug a moat just north of the fjord and placed a gatehouse there.
And in the east, I placed two gatehouses just east of the narrow passage from the keep (one facing NE for traffic to the quarries and mines, and one facing SE for the woodsmen / farmer traffic.) I then built a wall to the NE, cordoning off my production from any enemy incursion.
I built clusters of towers with ballistas at every gatehouse with another cluster of 5 towers at my keep and then tried to produce as many archers as I could to staff the towers.
When the '87 and '92 attacks arrived, the invaders sat perplexed for a while, then came in waves. They never even breached the outer defenses and never got by the moats.
So while facing 1500 attackers seemed impossible, it was really quite simple. Total anti-climax.
I built no defenses to the NW, as I saw that water prevented invaders from starting from this point. Everything had to come from the south or east.