Right, I promised doors, and doors you shall have. In the original TSR maps the doors were squares. Here's an example door that I have knocked up for you:
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Note that this is a 50px by 50px image so it fits the grid size I am using. You can get this for yourself by right-clicking the image->Save Image As... Place it somewhere you can find it easily.

Now when we start placing doors we'd best create a new layer. So go back to the new layer button again. Name the layer Doors and make sure it is selected.

Now go to File->Open... and find where you saved that door image that I posted above. Open this. Select all using Select->All or ctrl-A and copy using Edit->Copy or ctrl-C. Close the door image and click Don't Save when prompted - you haven't changed it anyway.

Now return to your map. Click the map window so that it has focus, and go to Edit->Paste or ctrl-V. Now you'll have a door image in the middle of your canvas. It will almost certainly be in the wring place. To move it, pick the selection tool from the toolbox:
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Now if you hover the mouse over your pasted door, you'll get a little cross-hair with a little four-directional arrow on the bottom right. When you hover the mouse away from it you'll get an anchor on the bottom right. Make sure you don't have the anchor and click and drag the door image to the position you want. The grid will help to an extent, but you may need slightly finer control. If so, place it roughly, then move it into its final position using the arrow keys on your keyboard. Once it is in the right place, move the mouse until you get the anchor symbol and click to place it. You can also place it by clicking the Anchor layer button in the Layers dialogue.
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Now the big square doors are all well and good, but they're not very realistic and they don't show which way the doors should open. Here's a different door that conveys that extra information. I know that's not really cricket, but it was requested in another thread, so here it is:
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You can place this in just the same way as the other doors, though you will need to use the arrow keys to get the placement right.

Using these it is straightforward to do all the doors you need to place:
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Note that you can rotate the images you want to lay down using the rotate tool from the toolbox. Just select the rotate tool and click the object before you anchor it. Equally you can use the Flip Tool to flip an image horizontally or vertically. I did this to get the double doors at the top of the map. Once you have manipulated the object into the orientation, go back to the selection tool to move it and anchor it.