What are the exact requirements to make a spear shop spawn?

I'm new to Kingdom: Two Crowns, and I had issues consistently making the spear shop spawn.
I've looked a bit and according to the best guide I found requirements would be:

• 2 stone-age walls defending both directions (4 walls in total)
• which needs tier 5 camp
• which needs stone mine (isle 2)

This feels somewhat wrong to me, I think on one isle I met these requirements without a spear shop spawning; and on another I had a spear shop without meeting them (only one wall on the right). I could be wrong as I was quite confused and tried lots of things to discover the game...
Not sure if it's relevant, but I'm playing co-op split screen (I read somewhere co-op is still a bit buggy).

Can someone confirm or refine these requirements ?
Is there randomness in shop spawning ?

Solution 1:

After a few more games I can say the requirements are correct.
In some instances pikemens (an ninjas in shogun) popped even without having all walls upgraded to stone, so always start by upgrading the outer walls.
Rule of thumb: Reload! If you think a building should have spawned and did not, do not hesitate to reload the game. It helps a lot. Also cut down trees between reloads, sadly sometimes you have to remove vagrants camps or merchant.