So everyone must have noticed the lane creeps grow bigger and stronger as the game progresses. Hope this post helps you understand in more detail the process of creep upgrades. Read on...
The spawned creeps (ie. Treants, Druids, Glaives etc) undergo a number of upgrades throughout the game to allow the game to end faster. There are basically three different upgrades that happen:
1. Periodical buff to HP and Damage
2. Increase in creep numbers
3. Buff resulting from destroyed Barracks
Periodical buff to hp and damage
This upgrade happens every 7:30 minutes. This is indicated in the creep's portrait like below:
Thus, after 15 minutes, the number will be 2 to indicate to levels of upgrades that have occurred.
Note that this upgrade only happens to Treants/Ghouls and Druids/Necros, not Glaives/Meat.
The buffs that happen are:
+1 min & max damage for Treant/Ghouls
+2 min & max damage for Druids/Necros
Illustration: Here's an illustration to help you better understand.
Initial HP of Treant: 550
Initial Dmg of Treant: 19-23
After 22:30 minutes (3 upgrades), the creep would have:
Increase in creep numbers
If you've been playing for a while, you might have noticed that the number of creeps increase as the game goes on.
Treants/Ghouls increase by one at minute 17:00 and increases every 16:30 minutes to a cap of 6 creeps.
Druids/Necros & Glaives/Meats increase by one at minute 45:00 to a cap of 2 creeps.
What it means is that throughout the whole game you can only have a maximum of 6 Treants/Ghouls, 2 Druids/Necros and 2 Glaives/Meats.
Thus, creep increments for Treants/Ghouls will only occur at minute 17:00, 33:30, 50:00.
Creep increments for Druids/Necros & Glaives/Meats will only occur at minute 45:00
1:30 - 3 Melee, 1 Ranged (1 Siege)*
17:00 - 4 Melee, 1 Ranged (1 Siege)
33:30 - 5 Melee, 1 Ranged (1 Siege)
45:00 - 5 Melee, 2 Ranged (2 Siege)
50:00 - 6 Melee, 2 Ranged (2 Siege)
* Siege initially spawns at minute 4:30 and every 7 waves
Note: This does not apply to -lm or -rd modes.
Buff resulting from destroying barracks
When the barracks at the opponent's lane is destroyed, the creeps in that lane will get an upgrade. In order to calculate the buff to the creep, the following formula is used:
HP = 150 + 9a
Min Dmg = 17 + a
Max Dmg = 21 + a
Armor = 1
HP = 175 + 8a
Min Dmg = 20 + a
Max Dmg = 20 + a
Armor = 1
Glaive Thrower/Meat Wagon
Dmg increases from 35-46 to 51-62 when ranged rax is destroyed
where a = Upgrade Level
If after 22:30 minutes, the middle lane's barracks (for Treant) has been destroyed, the Treant's stats would be:
HP = 580 + 150 + 9(3) = 757
Dmg = (22 + 17 + 3)-(26 + 21 + 3) = 42-50
Armor = 2 + 1 = 3
Mega creeps happen when all the barracks in the opponent's base are destroyed. All your creeps will gain a level 30 upgrade.
Note that if middle lane has the last barracks that is destroyed, top and bottom lanes' creeps would instantly upgrade to Mega level while the middle lane's would only have upgraded creeps when they next spawn.
Inspired from MrX's guide on Neutral creeps at DA.