A wagon always starts on the surface. A 1x3 stair should be dug down immediately adjacent to it. A refuse pile and a wood pile should be designated one square away from the wagon (to leave room for later walls). Then trees designated for chopping (if present). A pasture setup to contain the animals until they can be moved underground. Designate a dump.

Stage 2.
Wall in the wagon. Build a 4x4 gate and tie to a lever below. If safe build a trade depot.

Stage 3
Wall in all the area. Build an outer gate tied to a new level below. Expand the citadel to a second floor with fortifications. Plant useful above ground crops. build an above ground plant stockpile for gathering.

Underground aquifer dig. : Room for embarked goods, Carpenters shop, mechanics shop. Corkscrews, Blocks, mechanisms, pipe sections. Small farm.