Singleplayer maps generally require more versatility/replayability than multiplayer ones because things get more repetitive faster.
For now I see little content from the start.

1. Fire and Cold Towers share the same icon.
2. Ivey can't use her Summon Archer spell from level 1 because she hasn't got sufficient mana.
3. Poison Arrows' position before autocast is not in the proper place. Also, the icon doesn't have autocast borders. They can be made with: Button Manager v1.8.2 and/or Warcraft Image Lab
4. The Bow items have the same icon. Multishot share this icon as well.
5. Endurance Aura should have a passive looking icon. Can be made with the above mentioned programs.
6. Actually all Ivey's abilities are improperly positioned in the object/ability editor.
7. Summon Archer's icon should be that of the night elf one since that's what it summons.
8. The units in the barracks have no description and they are all the same (even the values, no role difference): Tooltip Tutorial and Creating Good Descriptions
9. Inferno Tower's aura is also named Endurance Aura as the frost one's.
10. Cold and Poison towers share the same hotkey.
11. Freeza and Ivey pretty much have the same abilities. Similarly the Blood and Archmage. All heroes have Critical Strike. IMO, heroes needs to be different. If you want Critical Strike to matter that much, it can be given by an item or a starting ability on all heroes which you don't need to level up manually.

Generally, basic gameplay mechanics. Could definitely use more replayablity stuff. Other than the heroes, the game is pretty much the same. Towers are too few to not use them pretty much all in one go.

Simple for now.

If you're not satisfied with the review and wish another opinion, contact the other reviewers or use the staff contact:
https://www.hiveworkshop.com/forums/staff-contact.692/