Current questions and answers regarding the map editor.

Sorry, I meant that any hero who visits this building receives +4 to Knowledge.

Here is a script that will work:

knowledge_building_heroes = {}

function knowledge_building(hero)
	if not knowledge_building_heroes[hero] then
		ChangeHeroStat(hero, STAT_KNOWLEDGE, 4)
		knowledge_building_heroes[hero] = 1
	end
end

Trigger(4, "object name", "knowledge_building")

in a simplified form.

Added 8 minutes later

Can I add my own ground texture for the editor?

Added 2 minutes later
And can I also create my own water in the editor, for example, toxic water?

Right-click on the map properties tree. In the list that appears, select Show Hidden. Hidden map properties appear. Find tiles, add a new element there. Click on the row in the Value column and on the ellipsis. On the left, create a new AdvMapTile object, and on the right, provide a link to the texture, etc. After saving and reloading the map, new painting textures should appear.

This way, you can even create a river of milk and banks of jelly.