One piece of advice

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Keep working on getting the Geisha House built, stay on schedule with this.

ichi

I've never even built a Geisha house!

The strategic unit side of the game never interested me much.

For SP, I guess my advice would be, get your unit balance right. Keep a balance early in the game between yari ashiguru as cheap garrison troops, and yari samurai (backed up by the odd archer unit) as your main battle unit. As your armies grow larger, try to include one or two of the cheaper cav units in every battle for running down routers, you can massively increase enemy casualties by doing so.

Meanwhile build toward a buddhist temple so that by the midgame you can churn out buddhist monks, they are the best combat unit in the game IMO. As a general rule don't bother with swordsmen, they are too vulnerable to ranged fire and always seem to take heavy casualties.

Also, specialize your provinces. Archers from one province, monks from another etc. Pay attention to the special bonuses of provinces, and try if possible to build up your best infrastructure in provinces with iron sands, they are the only provinces that allow you to build troops with armour upgrades.