Comment 3 for bug 1480935

The idea was the following:
If there are two buildings of the same size (small, medium, big) that are directly buildable, the smaller one (smaller soldier capacity) prefers rookies, the bigger one prefers heroes.

E.g. the Atlanteans:
The guardhouse and the small tower are both small buildings. Since the small tower can house more soldiers, it prefers heroes; the guardhouse prefers rookies.
Possible medium buildings are the tower (4 soldiers, therefore rookies) and the guardhall (7 soldiers, therefore heroes). The high tower is an enhanced building and therefore prefers heroes.

This was made for the following reason: If Atlanteans expand their territory, they'll probably use towers instead of guardhalls (cheaper, bigger conquer range, bigger vision range). But while no enemy is found, there is no necessity to send strong soldiers into the no man's land. To defend borders, a guardhall (with strong soldiers) is more suited.

When there is only one building of a size (Barbarian sentry, Atlantean castle), it depends on whether the building is rather "big" (used to defend borders) or rather "small" (so no fixed rule).

There is one exception: When you conquer a building, it always prefers heroes because at war, this choice is probably the best one.