You get about 0.1 seconds to pick your character before you're prompted with "Are you new?", and never get the option to select your character again.
It's a neat game, I had to restart and just hit enter as soon as it started and accept what I got to play. You need to fix that issue, it's extremely annoying.
Another issue is that there's simply too much to control and not enough time to do it. Maybe you should allow the player to hotkey some of their favorite spells onto their hero, so they can use them that way. You can just trigger it to cast the ability on the target that the player targets.
Here is my suggestion to improve the map:
Instead of using abilities for the hero controls, just use triggers to detect the player pressing buttons, and use that to control the heroes. This will free up space. Next, put the spells for the sanctum onto the controllable tower, so that they can be cast from it -- even if you just use the tower to cast the abilities, and the sanctum then actually uses them.
Have the tower remain selected by the player as the default unit.
Now, the player can use the controls for their hero, use hotkeys to cast spells, and right click enemies to attack them with the tower, all without having to switch units.