Best advice I can give with regards to creating maps is to get used to using the 'Gizmo' tool to position objects. It will allow you to position assets however you want. For instance, I can see many spaces between the tiles you've laid down. If you'd used the gizmo tool to position the tiles you could leave them with no space between them. If you prefer to not use the gizmo tool, then holding down the "T" key will force objects down to the table so you can push them closer together without being forced over the top.