Level is a statistic of Units. Units increase in Level by gaining experience. When a Unit's Level increases, it is called "leveling".Erf-b1-p065 Leveling increases a Unit's power by increasing statistics and granting access to new abilities. The Unit's power increases linearly relative to Level. The (theoretical) experience requirements for leveling are exponential, requiring croaking many more enemies (or enemies worth more experience) to increase a Level than the previous level. Although there is no maximum Level, levels beyond 10 or 11 are extremely difficult to attain. Because of the immense Experience requirement, surpassing Level 20 is very rare. The highest level ever seen in the story was Olive Branch at a level of 12. Jillian had only ever encountered one or two other units at the same level.
It is possible, though difficult, to gain levels by training alone. Artemis went from level 6 to level 7 this way, while managing a city: it took her 640 turns. She also trained up her knights this way.LIAB Text 50
The term "level" is also used as a statistic describing the defensive difficulty of a city. Improving fortifications increases a city's level; damage to a city's fortifications can reduce the level. The range for the level of a city is from 1 (weakest) through 5 (strongest), though some cities are considered to have a higher effective level in the form of defensive bonuses.
According to Scripture, analyzing one's performance, such as in combat or casting, is part of the process of leveling. Leveling in Erfworld might not just be an automatic process of killing things and earning points, like in a normal game, but a way of permanently retaining and reflecting a unit's actual training. A unit that self reflects often may well level up faster than a unit that does not.
It's possible that leveling is based on groups of 8. As an example: Bob, a Level 2 stabber, would need to croak 8 Level 2 Gobwins to reach Level 3. Alternatively, if Bob were fighting weaker Level 1 Marbits, it would take 16 to level. Alternatively if, via Luckamancy perhaps, Bob managed to croak a level 5 Warlord, he would immediately level from that single opponent.
If this is true, Bogroll would've gone from level 4 to level 6 when he croaked Ansom, as each 3 levels of difference between opponents accounts for 1 full level in terms of experience gain.
Stanley states that Gobwin Knob is the toughest defensive position in the known worldErf-b1-p23, which would follow that no cities have a higher level or have defensive bonuses and levels equaling 8, presuming that Stanley was repeating established fact and not boasting.