Suggestion on building and wall construction
I'd like to see buildings and walls built by workers and not a magical imaginary crew that pops in, builds something, and pops out again. Building should be similar to how the Elven Enchanters summon a Treant or Bolt thrower, they gather around in a circle and work on the unit until it is complete. While the unit is being "constructed" they are considered busy and can not heal or fight or do anything but summon (build).
So let's say you want to build an Elven Alchemy Lab (or Mill for Humans). You click on a worker and select build and it stops it's gathering and goes off to build. One battalion of workers will complete the structure in it's normal time. You can assign more workers to assist and reduce the time by 50%, another crew drops it to 30% and a fourth group drops it to 18% (more of a inverse log vs linear decrease where the more crews you add the less they will contribute as they tend to get in each other way.)
So you have the structure being built faster but those 4 worker battalions are tied up and can't gather during that time. This is a fair tradeoff; resources for building speed.
Implementing this also requires removing the "Rush" option for 1 crown (which is also an immersion breaker).
Now, let's say you want to build a stone structure or wall... well you have to hire an expert that works with stone! You have to have a Dwarven miner that costs a few crowns to obtain (here's your crown sink instead of the rush option). And much like wooden structures, the more crews of Dwarven miners working on a structure the faster it will get built (up to a max of 4 battalions)
Repair should work the same way.. click on a worker and select repair wall, building, or repair all. Dwarven miners would be required to repair stone as well.
Many RTS games work the same way because it makes sense to have the workers do the working.