That's a problem that can happen from time to time to a user. What you need to do is the following (assuming Chrome):
1) Open an incognito window
2) Open dominion.games
3) Enter your credentials (name/passwort) but do not click the login button.
4) Instead, under the Login button, is a relatively small line of text, with the entries "Kick" and "Kick and resign". Click either one. They server should answer with "Kick successful" or something similar
5) Close the incognito window and go back to your normal winow, where the spinning Castle should be gone (maybe press Ctrl-R in that tab to relaod), and login as normal.
If you make an error (like I usually do in step 3 by routinely clicking login), close the incognito window and start again (with a new incognito window) at step 1.