That's the most common build but I don't think it's the best build.
Delay shield, prioritize mulch is better imo.
But I have to go so I can't even explain extensively.
This is what I've been doing lately as well.
1. Shield I
2. Shamblers I
3. Ground Spikes I
4. Ground Spikes II
5. Mulch I
6. Spike Wave I
7. Ground Spikes III
8. Shield II
10. Mulch II + Ground Spikes IV
11. Shield III
12. Shield IV
13. Spike Wave II
15. Goddess of Thorns + Mulch III
16. Spike Wave III
Option 1 (against people with low AoE)
17-19. Entourage I - III
Option 2 (against people with lots of AoE)
17 - 18. Summon Shamblers II - III
19. Morale I
20. Morale II
I don't prioritize Spike Wave much because it is very low damage/per mana and typically you only use it when someone is trying to back up, or when you port and trying to catch someone running. I know that the range of the ability extends per level, but I tend to get Cloak of Night which helps cover the distance.
You could easily make the argument of swapping levels 1 and 2. If you get Shamblers level 1, you can summon both of them at the crystal which is nice. Plus, everyone hits level 2 so quickly, you rarely will know the difference. However, shielding at the crystal seems to have a more noticeable early-game impact since it guarantees you the XP flag.
There are a few advantages to leveling up mulch instead of shield:
1- You can Shield someone else and then Mulch for yourself to keep both of you alive. If you just level up Shield, you have to make the choice of protecting yourself or an ally. Against teams with high single-target damage (UB, Oak, Erebus) this may not be quite as desirable, but against teams with lots of AoE (Oak, Occulus, UB) then this becomes more desirable.
2- Keeping Shield low level means you always have mana for it, which makes you a cheaper build overall since you need less helms
3- Mulch marks creeps like Cirlce of Fire and Heaven's Wrath. Regardless of where you are in the map, any creep that's been tagged with damage from this ability rewards you with full xp and gold. You can kill creeps in the mana lane while capping XP flag, for example. This works especially well on Prison.
4- Mulch gives you hp, which has armor. It also does a little bit of damage (mostly only useful against Erebus).
5- Your opponents can't tell what level shield you have by looking at it. If you shield your UB, they'll probably assume that he has the highest shield available when he may not. This means they might back away with him because he looks scarier than he might really be.