Each unit (regardless of type) counts as 1. To attack an enemy territory:

Arriving army size	Attack threshold
1 – 5 units	Must be ≥ defender count
6 – 10 units	May be up to 3 fewer than defender
11 – 15 units	May be up to 4 fewer than defender
16 – 20 units	May be up to 5 fewer than defender
21+ units	No restriction

⚠ A Siege Engine is always required to attack a city that has a Castle.

When an automaton army departs a territory it must leave units behind:

• Non-urban territory: always leave 1 unit behind.
• City: leave a number of units equal to the city's Tax value.

Which units to leave (in order):
A. Footmen  →  B. Archers  →  C. Siege Engines  →  D. Cavalry

Automatons spend at the row matching their exact coin count (or highest row ≤ their coins):

Coins	Purchase
5	2 Footmen + 1 Cavalry
6	1 Footman + 1 Archer + 1 Cavalry
7	2 Footmen + 1 Archer + 1 Cavalry
8	2 Footmen + 2 Cavalry
9	2 Footmen + 2 Archers + 1 Cavalry
10	1 Siege Engine
11	1 Footman + 1 Siege Engine
12	1 Castle
13	1 Cavalry + 1 Siege Engine
14	2 Footmen + 1 Castle
15	2 Footmen + 1 Cavalry + 1 Siege Engine
16	2 Footmen + 1 Archer + 1 Castle
17	2 Footmen + 1 Archer + 1 Cavalry + 1 Siege Engine
18	2 Footmen + 1 Cavalry + 1 Castle
19	2 Footmen + 2 Archers + 1 Cavalry + 1 Siege Engine
20	3 Footmen + 1 Archer + 1 Cavalry + 1 Castle

No castles left in supply → buy 1 Crown Card + 2 Footmen instead.
Missing a figure type → substitute 2 Footmen ↔ 1 Archer. Keep leftover coins.